I can't recall, was your wife in a suspected MLC? Does this impact your thoughts on her actions?
Not to excuse them, but to see her whole cheating as part of something she was going through. A symptom of her greater issues. Do you think it was wildly out of character for her up until that point in time?
If you do think MLC, do any of her other symptoms seem to still be around, or have they changed too? How she dresses, anger, depression....