It's just that this is a very bad behavior. As long as you are his girlfriend, then you shouldn't do such a thing, but if you are really careless, then I think you should tell your girlfriend frankly, because he will know sooner or later. After all, it is his best friend and you. Tell him sooner or later, let him see what he thinks, and then apologize to you today. If you still like him, please. I can really apologize to him and see if he can forgive you, but if you have no feelings for him, just let him go, which will make him less sad.