An OT is generally someone who can tank an add, or eat a special that hits the target with second threat. If there's one target, they are ideally DPSing. If I were OTing for someone else, I would be in DPS gear and Berserker stance trying to do decent damage, not in my tank gear while doing about zero damage and in danger of pulling aggro. It's best to have the OT DPS on single pulls or bosses unless they are specifically needed, in my experience.
If it is really bothering you have your hunter start the fight off with a miss-direct to you after you attack. Then if the OT is still pulling the mob you know he is using taunt.... which he shouldn't be doing.
Question. . . since in this situation both tanks are on the same mob, there really is no "off" tank right? So why is he using a shield? Why not go in with his tanking gear in case all heck breaks loose and he ends up needing to tank, then after he's in combat switching out to his 2H or DW so he can do more dmg and be in Battle or Berserker to cut down on his threat?