If she likes her future MIL, and wants to invite her, I think that's fine. But if she wants it just to be the two of you, that's fine too. I don't think there is any "right" or "wrong" way to shop for a bridal gown. The bride should be able to ask, or not ask, whomever she wishes to go with her.
I paid for my daughter's wedding gown and went shopping with her several times. She asked her future MIL to go once with us, and her maid-of-honor and bridesmaids went along too. We had lunch while we were out, it was a very nice day. The last time we went, when she finally found the "gown of her dreams" it was just her, myself and her maid-of-honor.