What's the etiquette on asking somebody (a friend of my sister's) where she got her dress? I loved the photos I saw and would like something similar, or to know where she got it, but I'm not sure if it's rude or invasive to ask somebody I don't know very well, since it's (a) sort of asking how much she spent on it and (b) I don't want her to think I'm hijacking her style or anything!