What will you do if you learn that this guy is cheating on his SO?

You are family friends of this couple. She is very beautiful, had many chances to sleep around and more but is very faithful. He in turn sleeps around when on business trips. He sometimes visits brothels as well. You know because he sometimes boasts about all this. Should you tell her?

Never will I tell her. She should find out on her own like Tiger Woods and Elin.
I will tell her when I meet her next time
I will send an anonymous letter to her and ask her to hire a private agent
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What will you do if you learn that this guy is cheating on his SO?
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