Bridesmaid Dress Issue

My bridesmaids all ordered the same dress. One of my bridesmaids ordered the largest size available and it didn't fit (I thought this was odd, because although she is large chested, I was shocked it didn't fit). Everyone else's fit, but because they all will match, I told her to let me know asap as they would have to return their dresses within 30 days and we could figure out another option. She insisted she would get it altered/lose weight. So far she has lost a little weight (5-10 lbs). I have asked her a couple times if she has tried on the dress and she keeps telling me "not yet" and that "she needs help zipping it." I know for a fact she hasn't gotten it altered. Not really sure how to approach this as the wedding is in under 3 months and not sure what to do if she can't wear it.