When criticizing any creative work, whether it be visual art, music, film, dance, performance. You should be firm, critical and constructive. Be honest, but don’t tear the person apart for their work. Suggest where they need to improve and highlight the positives.
Now I’m not saying that you need to baby them, be very honest about what you truely think, but so brutal that you bring the person to tears and completely turn them off creating work. Ensure that whatever your criticism is, make sure you help them to improve, grow, strengthen. Rather than break them down and leave them in a crumbled heap.