depends... what colors look best on her? Is she blond, brunette, redhead, etc? Pale, tanned, etc? Are you going to light the background at all, or leave it totally black? Are you going to rimlight her.. or at least use a hairlight?
Let her pick.. anything but black.. and probably not a dark brown or dark gray.... go for contrasts. What color are her eyes? Matching her eyes can sometimes be a good choice.. or contrasting with her eyes if they are dark.
I would have suggested dark clothes, and certainly not bright clothes.
The reason is that when you are looking at a photo, your eye naturally goes right to that brightest areas and/or the areas of greatest contrast. So if the shots are meant to be portraits (she is the subject), then by matching (keying) the clothing to be similar to the background, you let the person's skin/face stand out and become the place where the viewer's eye is attracted most.
So if you have a dark background and dark clothing, that is a low-key image. If you have a bright background and bright clothing, that is a high-key image. Both are very good techniques for portraits.
Now, if the subject of the photo is the clothing, then you want the clothing to contrast from the background.