umm they're doing 5 man non heroic content.  no need to get so elitist about specs... seriously

but no they don't have to take him if there's a better person available.  It's his choice if he doesn't want to spec for pve.

A warrior who has 'epic gear' and tells mages to wait for 5 sunders is a noob warrior, anytime. My guild's mages all crit frostbolts for 5k frequently, but our maintanker can still hold aggro without the mages dying (unless there are other mobs, then it's their problem ).

Don't group with the warrior unless he's up for DPS. Screw him as a tank, since he's so stubborn about it. You will find that a skilled tanker will be pressing enough buttons to hold aggro with 2 sunders or less.

If your guild has a problem with him, throw him out. You can be sure that many other guilds won't accept him as a tank.

I've seen Fury warriors who can tank, but of course the damage they take is much much higher than if he were a Prot tank. And if he can tank until healers run out of mana and the boss still isn't dead, either your dps is not high enough, or his threat isn't high enough for dps to take over.

Just kick him from the guild, ignore him, and label him as a bad tank. It'll benefit you more than him.