Keeping secrets

[college kid]

Absolutely tell. Knowledge is power. Period.

[OtisTheMarsupial]

I would tell the friend that their partner is cheating if
1. I knew for a fact that there was actual cheating going on and
2. I thought the friend would believe me and
3. I thought the friend wouldn't react badly towards me

I would not tell if I only heard it second-hand. I'd have to know (see it in action, hear from multiple sources, etc.).