It's possible your acquaintance knew your GF was cheating on you, but to tell you would mean risking blowing up his own relationship for your sake. And that's an unreasonable expectation to have of anyone other than a true best friend.
The difference is you have no cards left in the game. You can flip the table over and walk. And you'd be giving the acquaintance the opportunity to make an informed decision about his relationship also. They might still work it out, but he should get to have a say.
The difference is you have no cards left in the game. You can flip the table over and walk. And you'd be giving the acquaintance the opportunity to make an informed decision about his relationship also. They might still work it out, but he should get to have a say.