First things first, your GF did you wrong, she betrayed you on every level a human being can betray a person and SHE needs to make this right.

The first thing she can do to make it right is to take herself out of the situation she is in. What is to stop Jake from telling her to bend over his desk "or else" pick up that pen "or else" meet me at X place "or else" she has a sword at her neck and she isn't doing anything to remove it. She needs to quit her job and find something else that takes her away from his control. The other thing she needs to do is compile a file of everything that has happened as a safety net against jake in case he tries something dirty. "if you don't leave me alone then your wife gets it all and so does HR!"

Then after that, both of you need to get yourselves into therapy, individual and couples if you want any hope of getting past this. it will take time, it will take consistent behaviour on her part and what ever you need from her, unlimited access to her phone and social media, a tracker app on her phone and a gps in her car (she must not know or she could take evasive actions) continued therapy for both of you, etc.

Maybe after that you will decide you can't do it, you tried and you just can't, maybe you will see that slowly you can get back to what you where before. who knows. but you can't get anywhere if she is still working with the guy she cheated on you with, especially if said guy has power over her and can get her to sleep with him again if he threatens her job safety or a promotion.

Good luck op, I don't envy you. Maybe spend a few nights at a hotel to clear your head and take the time you need to think, just don't drink yourself stupid, you can't think if you get drunk.