I think, Dub at the end of the day, it could be either way, like you said, she said she did want to break up at some point, or at least thought she did, it could be guilt, but it could be guilt from cheating, or from feeling like she let you down by letting this happen, if it did happen as she said, the thing you've really got to do, as much as it might not seem a rational reasoning, is to be there for her, if she did cheat, and did lie about it, thats something that does happen, more nowdays than ever, but at the same time, if it did happen, as she said, either way, I honestly think she needs your support, and thats all you need to give her, don't go chasing this other guy up, you really need to focus on her, and get her back to a confidence level and get her eating normally and feeling normal again. To make this sort of stuff up after an affair, or to have it actually happen, makes, requires, and obviously creates a big psychological trauma for her, and either way, I get the feeling you love her to bits, and really want to be with her. Just be there for her as much as you can, try and get her to see someone, maybe on her own for a start and then with you, and just do what you can to aid her to regaining normality. As much as people say it could be a lie and cheating, it could be, but to have the weight loss, stress, trauma etc.. that you have said she's been going through, thats a LOT for someone who merely had a quickie affair with a colleague. Worst comes to worst, when she's feeling better you end up going your seperate ways, but if it was me, i'd be trying to make sure she was ok, no matter what, as said above, one thing can push someone over the edge, if she is in a mentally unstable state, you dumping her and leaving could be the final straw for her. Not that you should stay in any guilt of "What if i leave.." circumstances, but you seem to love her to bits, just do what you can do, be there for her, don't go chasing that guy, whatever he's done, the damage has been done, any contact with him, isn't good, just move forwards, try to help her regain some normality, and let her know you're there for her.
Just my 2 cents mate, REALLY hope it can all work out for you both.