Despite being 50, did they still have the sprout icon?
Regardless of that:
1. If they were doing nothing but flash and shield lob, whoo boy, that's bad. Without chat logs, it's hard to pin down how hard I'd condemn because there's a difference between "This is better than my combo" (just plain ignorance) and "YOU DON'T TELL ME HOW TO PLAY!" (just plain jackassery) and "Lulz, Imma Captain America this dungeon" (just plain trolling). With the information available, he was flashing for aggro and just shield lobbing for the most part, that seems like troll or still ignorant noob. With the speed of leveling and lack of running dungeons (especially if they've been doing more quests than MSQ), it's possible they're still ignorant rather than trolling. That's a fairly generous serving of doubt benefit, but I can see the possibility.
(Sidenote: I'm particularly confused, however, why so many in the thread were talking about all the other abilities he would have at 50, but wouldn't have access to in that dungeon. That seems irrelevant in the situation.)
2. If during the run it wasn't clear if they were a legit noob doing noobish tanking that needed some guidance, the better way to establish a mentoring position to them would have been to tell the toxic healer/dps with the "drink bleach" macros to knock it off and take the stand in party chat rather than reporting them after the fact. Once you do that, if the player is new and willing to listen you're going to be better endeared to them and they're going to be more willing to listen to advice and open to establishing a mentor relationship with the player that stood up for them.
Waiting until the last boss to kick seems petty to me, though I also don't know how long it took you to get there and I don't recall how long you have to wait before you can initiate a vote kick. If it seemed like they were trolling or refused to listen, I agree with the sentiment of kicking them sooner if the option is available rather than waiting til the last boss.