With regard to alleged double standards - sometimes on here it happens, yes. However, I think more often what happen is an appreciation that these situations are not symmetrical. It is still the case that the majority of "wife work" and child care is done by the woman (in a hetero relationship). It's why it's called wife work. More often than not, it's the woman who is the SAHP. Not always, no, but often.
Thus when men have affairs, it tends to be them working outside of the home, meeting a young, attractive colleague who hasn't had children, moaning that their wife doesn't understand them anymore, and doing the dirty because they reject or feel left out of family life. in that situation, the sympathy tends to be with the woman, because she may well have given up her most youthful and vital years to have a family which the husband, in having an affair, effectively rejects.
When a woman has an affair, it is often (but not always) a form of escape from drudgery or even cruelty. Women are socialised to be people pleasers in a way that men generally aren't, so they'll put up with a lot of crap and make choices that are not right for them in order to please others. Their affair is an escape, perhaps before they feel they've given up the best years of their life to someone who may not appreciate it.
No one, or at least very few people, would condone an affair. But they might well view a woman having an affair with slightly more sympathy because they understand the complex motivations behind it.
It's not double standards - it's recognising that the situations are not symmetrical. You can't simply gender swap and cry "sexism" unless you've thought about whether the situations are indeed parallel. The friend is wrong to have an affair, but let's face it, we've all messed up at some point, even if not necessarily in that way.