It really depends on the relationship with the quilter. Do they actually want an honest opinion or do they just want to show what they've done? Can they handle constructive criticism? or accept a better idea?
I have a hard time showing my mother things because she automatically starts looking for what's wrong with it not what's right, but that's the way her mother was and any little imperfection had to be corrected. For a very long time it hurt my feelings something awful and I vowed to never make anybody feel that way! Therefore I'm very careful in what I say and always try to be encouraging. Now I only show her things from about 6 ft.!
I do hate it when someone shows something and points out every little thing . That's not good for their confidence.
So I guess you can say " Oh I just love that fabric" unless you don't! Or maybe comment on the pattern.
OK, I think I'm just rattleing now!