Let’s look at it from his girlfriend’s point of view. She wants to be monogamous and he has agreed to do so.
So he’s seeing you. Is he disclosing this to her? Is she OK with you being friends? How do you suppose she’d feel if she knew you were waiting and holding out hope to return to a sexual relationship with him?
Depending upon how you answer the above questions, is he demonstrating integrity through his actions?
That is a very good point @catfamiliar
I had not thought about it from that angle. I did ask previously if she was okay with us being friends. He said she was okay with it, which I found surprising because most people would not be. He also told me that she 'may' be open to opening their relationship at a later stage...which I also found unusual...because why would she ask to be exclusive and for monogamy if she wants to open their relationship at a later date. I think I'll ask a few open questions the next time I see him as a friend and literally listen to what he is saying. I put a boundary in place a while ago asking him to not discuss personal issues about his relationship with her. I felt it was weird to know deeply personal things about a person I have never met or even chatted to.
I do try to see beyond his diagnosis, but the more I read about it the more I question. I think you have pointed out some thing obvious which I had chosen to ignore up to now, thank you for opening my eyes. I do not want to be engaging in such behavior if he is being disloyal or lacking in integrity. Cheating is a firm boundary for me - and he knows this.