Depends on the situation. If you're hitting DPS checks and wiping because of it, then something needs to change. I would still address it politely and see if they are open to a solution. The only reason to boot them is if they are unwilling to learn or progress and are holding the group from completing the task. In my experience people only get defensive if you are aggressive about how they play to begin with. Don't be a jerk.

If it's just a standard DF dungeon run and the DPS is low, causing slower completion, it really isn't the end of the world. You'll still get it done, some people just have little to no patience. It's Duty Finder, suck it up. See if they are open to constructive criticism, but of not, just push through, finish the Duty and move on.

If it's something more difficult like an Extreme or Coil, then they probably should have learned their job before attempting it. However, if they are inexperienced with the fights, they may be focused on learning mechanics and staying alive more than damage, but may be embarrassed to openly say so. It isn't always just because they don't know their rotation.

Again, I would still try to see what the issue is, as they may just be missing something that is an easy fix, but higher level content becomes a less forgiving situation.

I don't think an immediate "learn to play" and booting them is ever going to be a productive experience for anyone.