#1: Dump him
#2: Dump the friend
#3: Take the kids
#4: Look for another man.

My fiance, even when we were engaged cheated on me. Yes, we had an open relationship but that doesn't mean he should date other girls and keep it a secret for months, that's considered cheating. The ONLY reason I put up with it multiple times was because all of his relationships were based on online romance only and he never cheated on me physically (aka offline). There's something about it being online that makes it easier but harder when it's offline.

A few months ago, he told me the REAL reason why he did those things. Apparently he had the misconception from somewhere that he MUST have relationship experience prior to marriage. He just couldn't believe that the first girl he dated ended up being his soul-mate, so he felt like he was inexperienced. You can bet that I told him that was a stupid misconception. He agreed and just said that instead he was the luckiest guy on Earth, where other men would spend years dating and being in failed relationships to find the "One", he found the "One" on the first try <33

Anyways, I tolerate cheating if it's online only and if it's usually before marriage. If it's physical cheating especially after marriage, there's no room for negotiations, I'd dump his sorry ass.