This is why I'm being brought along... to discourage any idiocy... I may not know a good dress, but sure as heck I can spot a bad one.
Your best service might be to prevent her from spending twice as much on the dress as she really has to spend. They really can be a lot.
Also have her be prepared that the wedding dress industry wants women to feel fat. Her wedding dress size will be a full two sizes bigger than she normally wears. Sonya's thin, and she was having to try on 14s and 16s. It's "European sizing". The psychology is all wrong. If you want me to buy an expensive dress, show me a size 5 I can fit into.