Tell a friend their partner is cheating, or stay out of it?

This is difficult, but I think you should stay out of it. Unless you have absolute proof and a very close relationship, getting involved usually just creates more problems.

You have to tell them. Staying silent makes you complicit in the lie. Your friend deserves the truth, even if it's painful.

Tell them. Honesty is important in relationships, and your friend would want to know if someone they trusted was betraying them.

Stay out of it. It's their relationship, their problem. You might lose the friendship by getting involved in their personal business.

You must tell them—friendship means being honest even when it's uncomfortable. How would you feel if the situation was reversed?

Better to stay quiet unless they specifically ask you. Sharing this information can damage your relationship with both of them, and it may not be as clear-cut as it seems.