BM dress alteration drama w/poll!

So, I got the call that my BM dresses came in! So, I called and e-mailed all of my girls. One of them e-mails me back and says that she wants to take the dress to get it altered elsewhere. The problem is, I added straps and there isn't enough materials from her dress to make them. Also, her dress maker doesn't know how the others are wearing it. Should I just let her do whatever? ADVICE!!! Please!!!!

P.s. This is also my FSIL..and her mother has been saying for months that she should do this...and I haven't gotten upset b/c I didn't think she was serious.

Also, one of my other bridesmaids is coming from CT to get it altered at the store b/c of the strap problem!

Also my SIl is pregnant with tons of issues with this dress and she is still staying with the store that we bought it from as well!