Stay OUT of it.
It's not your business, it'll ruin your friendship. But that doesn't mean you should say silent.
TALK to your friend. Remember, you're not there to tell him/her what to do, you're there to help them make the right decisions. Approach they/them (

) in a friendly manner and let them know you are aware and they should let their partner know/stop cheating immediately. If your friend lashes out or refuses to do so, leave it. That's they/them's problem at the end of the day and you tried your best, that's what matters. Because cheating never has a good ending, someone is gonna end up getting hurt.
My inbox is always opened. Let me know how it ends!