I'm a bit biased when it comes to Harrisons to be honest. I had a cockatiel who kept getting chronic respiratory infections year after year. I was told by my vet that the reason was that she was malnourished and had a vit A deficiency which made no sense to me. At the time I was feeding her a fancy blend of seeds, commercial pellets and some table foods. But the fancy seeds, she would pick through and the commercial pellets, the vet told me, are mostly filler, dyes and chemicals. I thought he was just trying to 'sell me a bill of goods', so I passed on the Harrisons. I took the anti-biotic for her instead. But she didn't get any better. I called them back several days later and the vet asked me if I tried the Harrisons (he gave me some samples) and I said no. He told me to try it. I did and a week later she was fine and never got another infection.
You can even tell with Skittles. His colors are so much more bright and vibrant and I know its the Harrisons.
As for your gf, I would highly recommend you have your gf get socially interactive with your sun. Suns have a tendency to become overbonded and if they are not properly socialized with others then they will attack anyone who comes near their 'chosen human' because they see that person as a threat.
The biting thing worked for me too. I don't react when Skittles bites (which isn't very often)- he will 'nibble' but it doesn't hurt, hes just being playful. But I use the "no biting" command with him when he bites and he's pretty cooperative with that. If you show a reaction (like freaking out or backing away) to their biting it will reinforce it. It sound silly to just 'let them bite', but it works for me too. Not showing a physical reaction and just saying "no, no biting" in firm words works great for me. When Skittles realizes he's not getting anywhere by biting, he stops.