I have a friend who's mom had a long term affair with the father of another classmate. Everyone in town knew and we grew up with the rumours. Her paretns did eventually divorce (for multiple reasons), I don't know if her parents ever addressed it directly, but A know the rumours and specilation messed with her head big time and she had some pretty messed up relationships for a wle before she met her husband.
I found as an adult that both my parents had affairs. I'd heard something as a child that made me wonder and it was always in the back of my mind, then years later a drunk relative let s ething sip. I brought it up with my mom and she was honest about it (my dad was dead at that point). Her theory was h wasn't going to advertise it, but she was never going to lie either. In my case though, it didn't rally affect me, my parents didn't divorce, there was no nasty gossip, etc.
I think in most cases, especially dealing with teenagers, it is best to sit them down and be honest.