I hardly ever down rate levels regardless tbh. If I didn't like it, I wouldn't, but I may mention it to them, as long as they ask me for my opinion first!
I'm not sure I like the idea of people knowing what you rated a level - it kind of makes you feel the need to please others, as opposed to give a true, realistic rating. I'd prefer to have ratings anonymously, that way I could be honest.
Saying that, if this friend's level was rubbish, but I knew they'd put a lot of work into it, I'd probably rate it up. Sometimes when rating levels, I may be more inclined to rate based on effort as opposed to the actual quality of the level.

