I would talk to her about what your plans are, but I wouldn't specifically make it about her weight. I would just say that you were thinking of buying robes for everyone and see what she says. If she seems cool with it then I would go ahead and buy robes.
Just ask if everyone would be comfortable with wearing robes if you decided on it, don't single her out just due to her size because there's slimmer women who aren't as comfortable either. You'll never know unless you ask.