I would say it's both a girl and an RSD thing.

Personally...I have a few dresses that I like and they are all knit dresses (made out of t-shirt like material). None of them are tight and only have elastic at the waist where it comes in a little but are not tight. I am super sensitive and it really is small things that can make clothing wearable or unwearable for me.

Same goes for shoes. I can't wear most gym shoes because they just hit my ankle in the exact wrong spot. I found a pair I could wear so long as I didn't thread the laces through the top holes...and then I bought two more pairs in other colors. I can also wear ballet flat type shoes because they are completely open on top and don't hit my ankle in the bad spots. Problem with those? I am SUPER sensitive to air movements and cold so if I'm going anywhere with A/C I need to have my ankle covered. I have a nice fabric ankle brace that doesn't look particularly sexy...but it does protect my ankle from the cold or air movements. So that's an option. I also need to wear sweaters or sweatshirts, a scarf, and gloves to protect my upper body for the same reasons.

I am far from picky from a style perspective...but my RSD makes certain things necessary. I NEVER wore fashion scarves before but now have quite the collection of them. I try to have fun with them now...but wish I didn't need them at all. Oh well...such is life.

I love when my boyfriend buys me things...but he's very aware of the sorts of things that I can and cannot wear. In time you will figure it out. Until them...maybe see if you can sort out what her needs are and what things absolutely do NOT work for her and then you might find something that works. But if sweats are her norm...maybe see if you can find a knit dress or one made out of similar material that looks nice. You'd be amazed what they can do with that sort of material these days.