N.C.
David's bridal is an excellent place to go. They have plus size and if there is any alterations they can do it.
My other suggestiong would be to have it custome made. You can find almost anything in the garment district.
Finally I would suggest online. If she does find something she likes. Order it in her size and a size larger.
I am plus size and work in fashion so if you need any more help send me an email @ ____@____.com