Usually if someone is doing something that's new for them, and positive, I'll try to back it up with some praise. I never criticise ... in the past, if I've had trouble and I thought my partner was messing up, I'd ask the coach/instructor why we were having such a problem, even asking what I was doing wrong. Heck, sometimes it was my fault.
The only time I might offer a little constructive criticism is if someone is having trouble executing a particular move, and they ask for advice or express frustration. An older friend of mine was having trouble with a very difficult waltz spin move, she couldn't figure out why she couldn't execute the move. I explained that if she would maximize the distance between her ribs and her hips, she might have the control to pull off the move. On the next try, she executed the move without stumbling ... but I would have never offered that advice, as benign as it might seem, unless she first asked.