Remember that Healers are people too.
Not all of them are experienced; they may be new to a dungeon, a raid, the game itself. They may not have played an MMO before. They may not have fast hand-eye coordination, they may have trouble reading instructions. They may have anxiety, fear of failure, or fear of groups.
But maybe they like healing. Maybe they like playing with others despite discomfort that may bring. They want to play, and maybe to learn, and the only way to do that is to try. But MMOs are a group effort, even if you play alone, and sometimes people need help. Please be a kind person. Please treat them they way you would want to be treated.