I'm not sure I understand. How would him doing a taunt make him get free headshots on other people?
edit: Is it like, he started the taunt from in cover, then while his visible model is still behind cover he can go out and snipe while his model is back in cover for other players?
If so, that's not really 'free headshots' but it is indeed a terrible glitch.
If I got it all right, you start taunting but can move nonetheless thanks to the slide glitch. As you move however, you´re technically still locked at the position you started taunting at. So when other players try to shoot you while you´re moving away from where you started taunting, you take no damage. Thus you can easily kill the enemy player.