Having dealt with a similar situation (my friend was far more beta though) I can tell you there's little you can do. You'll only piss him off and he'll want to hang out with you less if you point out how insane he's acting.
Just be happy for him, hope he breaks free at some point, and work on tightening your game to assume the now vacant spot as your groups lead dog.
Just be happy for him, hope he breaks free at some point, and work on tightening your game to assume the now vacant spot as your groups lead dog.