My cat HATES my girlfriend

I have had my cat for 2 1/2 years now, he turns 3 in September. He has never given me any issues, he’s been extremely well behaved and is always very well taken care of. I have a girlfriend of 14 months, he has always been a little hostile towards her, but really just hiding and occasional hissing, nothing too concerning and I thought it would get better with time. She moved in with me about a month ago and it has gotten so much worse. He now deliberately attacks her, and not just like if she tries to pick him up or something, he will literally stalk her and attack her unprovoked, she has scratch marks all over her legs, I feel so bad. It’s honestly affecting our relationship even. We have tried a couple things from light punishment to having her be the one to feed him and give him treats to show him that he can trust her and nothing is working. This baffles me because he is so sweet with me. I don’t know what to do.