Sorry to hear of your predicament.
I really don’t think he can blame his transgression on MS, or at least on his judgement. He did it because he could. Perhaps in the light of his diagnosis and therefore some aspects of his life path such as a loss of potency, did he do it for validation? That it proved to him that he was still attractive to others? That he’d still got “it”?
Easy to moralise: We can all go through crises of confidence during life and MS will sure as hell throws up some issues. Up to you if you can or want to work through it.
Once a cheat always a cheat nothing to do with m.s