I totally agree that you need to take the Green Cheek back to Petco, that is what is BEST FOR THE BIRD! Your friend can't even afford the $400 for the bird and has to "save it up", and that means that he cannot afford the montly food bill, and especially cannot pay for a proper Avian Vet (they need an annual Wellness-Exam every year with an Avian Specialist Vet or Certified Avian Vet that includes complete Fecal Testing and Blood-Work, that is how you keep ahead of illnesses/diseases. And if your friend cannot afford to support the bird, then he shouldn't have the bird!
***And a Green Cheek Conure should live into their 30's, and your friend's Conure likely died at 2 years-old because they used either Bakeware or Cookware, or had a small appliance that contained Teflon and that killed the bird (it's not just pots/pans that contains Teflon, it's pretty much ALL small-appliances, like hair Blow-Dryers, Griddles, George Foreman Grills, Microwaves, Convection and Toaster Ovens, Space Heaters, Air Purifiers, etc., basically ANY appliance that creates any type of heat, because they coat the inside components with Teflon or other non-stick coatings that contain Perfluorinted-Compounds such as PTFE's, PFOA's, etc. So it's not just Teflon non-stick coatings (Teflon is actually just a brand-name of one of the hundreds of these non-stick coatings)...It takes literally 1 little whiff of a fume from a Perfluorinated-Compound to kill a parrot/bird pretty much instantly, within minutes, and this can happen even if your bird is behind a closed-door and on a totally different floor of the house than where the bakeware/cookware or appliance is being used)...OR, if it wasn't fumes from bakeware/cookware or a small appliance that killed your friend's bird, it was most-likely fumes from something else, such as cleaning products, incense, a candle, any type of "Plug-In" fragrance creator, an oil or wax burner, etc. AND NO, YOU ABSOLUTELY CANNOT VAPE ANYTHING AROUND A BIRD!!! IT'S NOT AT ALL SAFE AND WILL KILL YOUR BIRD!!! Even Vape Juices, Oils, Waxes, etc. that are 100% Vegetable Glycerin will kill the bird, let alone ones that contain PG, which most or at least many do. The best way to think about a pet bird/parrot and what types of fumes can kill it or make it very sick is to basically assume that if something lets-off any type of fumes, odors, scents, smoke, vapor, steam, etc., then it might kill your bird and you cannot use it inside THE SAME HOUSE AS A BIRD, REGARDLESS OF BEING BEHIND DOORS OR ON A DIFFERENT FLOOR. So if you want to Vape, you'll have to go outside. Period. And you can use cleaners, but they have to be all-natural cleaners that are made from 100% plant-products/enzymes. Most grocery stores now all carry these types of cleaners in their "Natural" Section of the store...
****Just because you bought your Conure from Petco does not mean he is sick or that he's going to die, or has any health or physical/mental issues at all. I'm certainly not a fan of Petco/Petsmart either, and they do buy their young birds/parrots from large vendors who do not at all Hand-Raise their babies, but rather only "Hand-Feed" their babies, which is why the tag at Petco says that your Green Cheek was "Hand-Fed" rather than Hand-Raised. There is a huge difference between the two, as being "Hand-Fed" just simply means that they remove their babies from their parents when they are at least 2 weeks-old but no older than 3 weeks-old, house them in a large Brooder from that point forward, and they remove each baby 5-6 times a day to "Tube-Feed" them with a Crop-Needle, which takes all of 30 seconds, they quickly fill up their Crops with formula and then they go right back into the Brooder. Most of the vendors they buy their baby/juvenile birds from get no more human-interaction than this...There are some exceptions, as my own Yellow-Sided Green Cheek Conure came from a Petco, I would never have purchased him but he found me, he chose me, it was a weird situation and just happened by complete chance...Bowie was 13 weeks-old when I brought him home from Petco, I got a Hatch-Certificate from Petco with him, along with a Health-Certificate, which means absolutely nothing from Petco except that they will refund your money in-full within the first 3-days after you buy the bird, after that you can bring the bird back but you will not get your money back...I can't believe they sold a 17 year-old a living creature without carding them, depending on the state it's illegal to do that, so that might be your way out, having your parents call up Petco or rather go to Petco and tell them that they "just" found out about the bird, that you have been hiding him in your room and they just today found out he was there, and that when they found out that this Petco sold a living parrot to a 17 year-old kid and didn't even ask how old he was, let alone ask to see his ID, they were extremely angered and immediately wanted to bring the bird back, as you are only 17, you're still legally a minor who lives in their home and is still supported by them completely, and that it's not only unethical to sell a minor a living parrot, but it very well may be illegal in whatever state you live in, and that if Petco refuses to take the bird back and refund your money that they will get an attorney and involve law enforcement if it is in-fact against the law to do so...This all depends on what state you live in and whether or not your state considers pets/animals as being "Personal Property", in which case it's totally legal to sell a minor a living pet, or if your state considers living pets/animals as not being "Personal Property", but rather as living creatures that must legally be cared for properly by their human owners. In that case it's typically against the law in the state to sell any living creature to a minor...I would also complain to Petco corporate, because I do believe that their own store-policy is to ask to see ID when selling a live-animal and confirming that the buyer is 18 years-old or older...
But as far as your Conure dying/being sick simply because they came from Petco, that's just not true, just as many young birds that come from private breeders and especially from Parrot Brokers, as well as privately-owned Pet Shops and Bird Shops die as do birds from Petco. Your concern about your Conure's health shouldn't be because he came from Petco, but should be that no matter where a young Parrot comes from, when their first owner brings them home, they should go to an Avian Specialist Vet for a full Wellness-Exam that includes full Fecal-Testing to rule-out Bacterial and Fungal infections, Protozoan Infections, Parasites, etc. that ALL BABY BIRDS CAN GET from their parent birds, their siblings, and their surroundings, no matter where they come from. This first Wellness -Exam should also include complete, routine Blood-Work if the bird is old/enough and weighs enough to do a blood-draw, which a Green Cheek from Petco should be, as they are typically at least 3-4 months old at the time they arrive at the stores, and then also a full visual/physical exam by the Avian Specialist Vet...And this should happen within the first couple of weeks of you bringing the parrot home...And it typically costs around $200...Obviously your friend is not a suitable home for this Conure, as he can't afford even the cost of the Conure, let alone proper Avian Vet Care, and even proper monthly Food for him, and whatever killed his prior Conure at only 2 years-old may still be being used in his home. Whatever you do, thank you for doing the responsible thing, which is what is best for the bird...