Peter is causing trouble

So on top of all the issues with him and Sweet Pea and him biting fingers being stuck in the cage bars. He's now taking up biting my girlfriend whenever she is petting another rat...
He doesn't do anything like that to me...
I mean he could just not like her cuz she bops him in the head when he's causing trouble and despite me fussing at her for it she continues to do it cuz she's afraid to pick him up to put him in timeout when he's acting all mean.
He has never bit me. I don't know what to do. She doesn't want anything to do with any of the rats cuz she's afraid of him biting her....

Are you getting him neutered?

He's been neutered for probably almost 2 months now

It almost sounds like he gets jealous of the others, is the environment different? has there been any changes lately? Sense he is neutered it wouldn't have to do with hormones, just behavioral issues, or possibly a health concern.

What is happening before he bites her?Is he biting and causing blood or is it just a little nip?

Nothing has really changed much... I mean we are packing for a move... But nothing really has changed in or around their cage. They had just all gotten treats. And Peter was laying under his hutch and Buck was at the front getting petted by my gf. And then Peter just ran out and bit her. Hard enough to draw blood. Not a lot. But on her knuckle and you can see puncture wounds where his top and bottom teeth broke the skin....

Bopping him on the head. That is not good. Can you imagine getting hit on your head when you do something that is undesirable to someone else? She needs to start respecting him and his space. I highly suggest that she no longer goes into his space. If she wants to pet another rat, take that rat out of the cage and cuddle him. She needs to give him time to no longer see her as a threat. When she does go into the cage, make sure she always has a treat to hand him. She needs to become the good guy.

I fuss at her for the bopping. she's just afraid he's gonna really hurt Sweet Pea or Buck when he starts acting up...
Thank you for the advice jorats. We will try that. That's what I figured the issue was