Yes.
True, but if you need more practice than the average, you need to go practice if you're going to enter a team environment.
Here's a better analogy for what I am talking about. There is a basketball player who does not hit most of his shots. He throws the ball in a bad way and thus misses the hoop completely. He sees an ad for a basketball team signup where he will be teamed with 3 random people. It is made clear that this is for intermediate players. In addition, he is given the option to play in a game with a bunch of robots, where he can practice without needing to care about making his team pick up any dead weight. Now let's be fair and assume that this player doesn't know that he is very bad. The player signs up for the match with other people. He is thrown into the match and when he misses multiple shots, someone asks if he can try throwing the ball a different way so he can make some goals. The basketball player gets angry that someone dare not approve of his style of throwing the ball and claims that the person asking for him to contribute more to the team is toxic. This is not ok as he is not only expecting a carry, but getting mad when asked to help the team. This is obviously not a good thing for someone to do, but some people in the crowd demand that the team just accept that he is bad and pick up the slack. The referee (GMs) agree and say that the team is forced to play with this person and if they refuse they will be banned from playing the game for 30 minutes.
This situation seems very unfair to the three people matched with this person, but for some reason it's ok when it's in FFXIV.
I can see his buff bar when healing him. I know when he's using mitigation as I have played tanks before. (Look at my main class) If he is not normally this bad, then he can take my criticism and not do what he did next time. However, having seen the tank's reaction, I'd imagine that he is not just having a bad run or he would have realized that he wasn't using mitigation when I pointed it out and started using it.