It's less about using a 'you' question and more about phrasing it in a way that puts them on the defensive by making it clear you think they're bad. You're not asking them if they're trying, you're saying to them, 'No, you're not trying and you need to fix it'. Sometimes it pays to play a little dumb and just kind of... wonder out loud why the group's damage is so low, or why health bars are always dropping. People will be more receptive to feedback if it the conversation doesn't start with them getting slapped in the face with 'Could you try a little harder?'

For all we know, that healer thought they were doing fine. Obviously they need to be disabused of that notion, but it's got to sting if they think they're performing okay and someone just straight up tells them they aren't trying.