Your best friend's partner is cheating. Do you tell them?

Absolutely tell them. Your friend needs to know so they can make informed decisions about their own life. Loyalty to your friend comes first.

I would tell them. Honesty and integrity matter. Your friend is being deceived, and they have the right to know the truth about their partner.

No, it's not my place. Relationships are complicated, and getting involved usually makes things worse. If they find out, they find out.

Difficult question, but I think you have to say something. How could you call yourself their friend if you stay silent while they're being betrayed?

It depends on how certain you are about what you saw. If it's solid proof, yes tell them. But if there's any doubt, stay out of it.