why would your friend ignore this? is she afraid to be alone? or is it an economic issue where she has become dependent on him and cannot go it on her own?
infidelity in a marriage is completely unacceptable. if it was going on all along, he had no business marrying her or she him if he clearly was not ready to commit - never mind family members and their "expectations", because i know people give in to that. or if this was something that arose as time passed, i would advise seeking out a counselor because she needs to know what she has not been giving him that he apparently needs, whether mentally or physically. that's IF she really wanted to save her marriage.
she needs to confront him either to throw him out and start divorce proceedings or otherwise. she needs his full undivided attention and she needs to present the proof and ask questions. at least hear him out. it may be tempting, but she shouldn't start yelling. if he has a feeling bone in his body, he should tell her why and accept the responsibility for his actions. no sense dangling her along.
it's disgusting.
the only thing i wonder is if there are kids involved?