Well, breaking up would b one way to express how that offends u, but the reality is that he would likely just end up with her and you would feel like ur enemies joined forces. by staying with him u at least exact revenge on ur ex friend. But... it's an imperfect system.
Just because u broke up a few hrs ago, doesn't end all hope. If He is loves u he will be back. Mean while u feel like some one sucker punched u in the gut. Surround yourself with your comfort team and lean on them. How did he react? Over defensive and unable to maintain eye contact? Hiding something. Men love full eye contact, Even if they don't know it. But I would be more than ticked if that rumor was true. He owes u some reason. Why her??