Would you tell your friends fiance she's been cheating for two months?

My friend has been with her fiance for 6 years and has been cheating for two months. She was planning to leave him but then he proposed. He has no clue.

Her fiance is friends with my husband. It's been so hard not saying anything even to my husband. She's been seeing this guy for two months now and even brings her 1 year old with her. The guy is ok, but also thinks she's leaving her fiance. She has tonnes of hidden pictures of her with him.

My husband mentioned that my friends fiance has been "off" for a bit and wondered if she said anything to me. I said no.

It's hard because her fiance is the father of her child and literally the nicest guy ever. The only reason she cheated was because the new guy showed interest and she wanted to show her fiance that she was "desirable" if he didn't propose by their 7th anniversary. It just sucks all round.

I don't know what to do. I'm sure it will come out eventually but I know my husband will be upset for not telling him and I know that will drive a wedge between my husband and my friends fiance.

Edit: I told my husband over dinner and he totally understood why I didn't tell him earlier. He says he's going to talk to his friend about it and he said his friend has suspected for a while he's been cheated on.

Thanks to everyone who commented 🙏 It's tough but in the end yeah... I'd want to know if I was marrying a cheater!!!