My bridesmaid fitted in her dress when I got it for her, and then gained 2 stone is as many months virtually. The original dress was discontinued so I couldn’t get it in a bigger size. I had to buy a huge one off ebay and get my mum to completely fashion a new dress out of the material (younger bridesmaid dress was already made in that fabric).
I conducted myself very carefully and tactfully in this situation. Ladies generally feel very sensitive about their weight and I’m sure she will like to think she can fit in it, my friend was the same. I strongly advise that you are kind and gentle with her and speak with compassion instead of checking up on her eating habits on FB. It cost me over £100 and a lot of stress to fix my situation but I did not make my bridesmaid feel uncomfortable with it all.
If you do nothing and she can’t do her dress up on the day that would be an absolute disaster and a really petty thing to do. Please just talk to her properly.