I've been bit by a snake before and it really hurt. Never again if I can help it. The first step in not getting bit is realizing it's a snake. He's hiding under a 12 year shield waiting for the ideal chance to strike. Step on the shield, force him into the open and corner him. Catch him off guard during a happy conversation and ask him why he's badmouthing you to your girl. Ask him why he's so eager to hang out with just her. Those are going to be the difficult things to explain, unless he just wants to bullshit his way though it. Give him a chance to be honest with you and admit what he's doing. He'll likely use that chance to make excuses or be defensive. If that's the case snakes make nice boots.