“But she tells me that what often happens is she thinks she looks great and then sees photos afterward and is unhappy.”
There’s a pattern here, and a high liklihood this will happen again, as I’m sure you already know, whether she chooses a dress to her taste or to yours.
That’s a h***uva strain to put on a friendship!
If I were in this situation I would try to extricate myself by encouraging the setting up of a couple of appointments to try on dresses. A simple way to insure that this emotionally fraught task is properly delegated to a professional. Not because your advice isn’t good enough, but so that you can serve the appropriate role of supportive friend, rather than style arbiter.
I would choose one old-line independent dress shop, if there is still such a thing where you live, and one department store with a sizable dress selection like Saks, Nordstrom or similar. But you have great taste and know your local market, and will know what the right shops are for this situation.
If it’s feasible, you could go along with her to the appointments. If not, you could FaceTime with her during the appointment. (I FaceTimed with one of my daughters when she made final decisions on her wedding dress - an emotional and also hilarious story for many reasons that I’ll share some other time!)
However this goes, I wish you luck and sending good vibes for the health of your friendship. I’m looking forward to seeing pictures soon of the family wedding you have coming up!