The issue of players who only do FATEs to 50 and don't learn their class can easily be solved by forcing you to do each dungeon in order to unlock the next for each job/role. In fact, I don't know why this isn't enforced.

So if you switch to a new job, then you're locked out of all the dungeons up to the latest one you've done for that job.

Though playing my own Devil's advocate, this could be a giant nuisance for those of us who actually put effort into learning our jobs and would just be punishing us for horribly unskilled players.

My actual suggestion: Just leave. It's one thing if the person is simply unfamiliar with the dungeon... you can teach that and help them through it with minimal difficulty. But when a person can't play their class and are so lazy they weren't even willing to put in the effort of learning it before jumping into a high level dungeon... then they deserve to get left behind. Maybe before you leave suggest that they start off at a lower level dungeon and work their way up.