I wanted to bring up this discussion because I have had to deal with this a couple times? One of my friends was trying to write a song but I thought the lyrics were all over the place. It was missing a theme and it was hard to interpret. Another one of my friends is really bad at dancing (if you would consider that art). If you have a negative opinion about someone's work, how do you tell them? Should you tell them at all?
You should tell them, I agree too. As friends, it's your job to find the best way to tell them however. In a constructive helpful manner, so that it hurts the least that it can.
I feel the safest way to give con criticism is to try to praise the good aspects and as well as telling how they can improve the good parts even better first. Then you could go deeper on the parts of the dance that really need fixing.
You know, something like a good cop bad cop thing~