Should I warn my son that his antics risk a family split?

My son and his fiancée, both 30, have been together eight years and have a 10-month-old daughter. I thought they were happy and that he was like me in how he conducted himself. I get on well with his fiancée and last week we had a chat without my son or husband being there.

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A few things came out, which I am furious with my son about. I was offloading a bit about my husband and his fiancée told me she'd found out that my son had been at a party without her and spent the night with a female colleague, ending the next morning with them alone together. My son and this girl als