what would you do?

...Hypothetically speaking
You have a really good friend who dated a mutual friend of yours.
They live together and are very much in love. However your friend tells you they are cheating on their significant other with your cousin. Your cousin has no idea your friend is in this "committed" relationship and instantly falls head over heels for your cheating friend. What would you do? Would you tell your cousin? Tell your mutual friend that they are being cheated on or do you just stay out of it?