I don't agree with the "If you can't say something nice, say nothing" idea...that silence can be as hurtful as the snarky comment because it's obvious that you don't see anything that you approve of, especially if you've complimented someone else's piece....
There IS something nice you can say about ANY quilt, no matter what the quality of the work is...gush about the colors, pick one particular piece of fabric you like and go into great detail about how you love it, and where in the world did she ever find it? Most won't notice that you didn't compliment the actual quilt...they're going to be pleased that you liked something about it. Compliment how straight the seams are or how you like how the binding was done....find SOMETHING nice to say.
Constructive critiques are only that if they're invited...until then, they're ego busters....