if it is just a one night stand, and nothing more, then telling your friend will only hurt her feelings. men are biologically and emotionally constructed far differently than women, and for most, sex with someone other than the loved one is meaningless for him. Unless you know that this behavior is more than occasional and is unprotected, why hurt your friend over something that would not change his love for her? BTW: Am in my late 50's, and blew a good marriage over a silly infidelity on his part. He didn't love her, didn't know her, hardly knew her name - but he did love me, and I couldn't see that. Please be careful with your decisions, as many cannot be undone later.
There is no such thing as one night stand when you are married so if she/he does it does it the first time, they will surely do it again.
if it is just a one night stand, and nothing more, then telling your friend will only hurt her feelings. men are biologically and emotionally constructed far differently than women, and for most, sex with someone other than the loved one is meaningless for him. Unless you know that this behavior is more than occasional and is unprotected, why hurt your friend over something that would not change his love for her? BTW: Am in my late 50's, and blew a good marriage over a silly infidelity on his part. He didn't love her, didn't know her, hardly knew her name - but he did love me, and I couldn't see that. Please be careful with your decisions, as many cannot be undone later.