My best friend is poly but has committed themselves to a mono relationship. Or tried to. They asked their mono partner to open the relationship recently, and when the partner refused they started seeing someone else anyways behind their back anyways. I understand not wanting to hurt the mono partner by breaking up, but in the long term this will hurt the both of them even more. They just keep saying they’re not ready to break up with the mono partner because they live together.
Your friend is cheating on their partner. That is wrong, and has nothing to do with polyamory.
You can’t control someone else’s behavior, but you can choose how to respond. If you feel like you need to tell your friend’s partner that they’re being cheated on, you have that right, though it will likely destroy your friendship. If you feel like it’s best to keep out of things, that’s also your right. You may find the friendship drifting apart anyway if you feel uncomfortable keeping this secret or even tacitly endorsing your friend’s behavior.
You can try to advise your friend that if they’re unhappy in a relationship, they need to leave it, not put the other person at risk by cheating. But they may not listen. All you can do is decide how involved you’re willing to be in a situation that you know is unhealthy for all parties.