I probably can't say anything that hasn't already been said. Some people join parties knowing what's expected of them and some don't. Like, if I join a party on my Conjurer then I'm gonna by default setup to cure because I've come to realize that's what's expected of me. At the same time, if someone plays Conjurer for the damage aspect of it then they might not have the same mindset. If we're grinding and something goes wrong and I see a way to fix it then I'll address it but there are right and wrong ways to do that. Someone is way more likely to listen to you if you sound like you're giving constructive criticism rather than just blatantly telling them their playing their class wrong.

If they got to 30 without knowing what Provoke/Cover is, then they probably had no reason to use it up to 30 whether they were soloing the entire time (or PL'd... that would suck). I think that if you're playing Gladiator to DD with a sword though and you decide that you wanna join a party to make the levels go faster that you should realize you'll probably be expected to tank. Most parties simply won't let you attempt to DD.

Comparing the cases between Conjurer and Gladiator are fairly different though. A Conjurer has the ability to completely set up to deal damage and still perform well while a Gladiator can setup to DD but they're much better off setting up to tank.

The only other real thing you can do if you happen to come across one that refuses to tank is tell him that that's why you invited him. It's not like it's against the rules to oust him from the party and find a willing tank.