Just venting! But what to do??

OK, all attendants live atleast 25 hours from me & won't be town until the wedding (july).
G is having, as his only attendant, be his sister this July.
Her dress coordinates with the BMs, but is a different style etc.
Told her that her dress has arrived in my town (where I placed the order) & that I'm getting ready to mail it to her.
She replies "I plan to come in early july to get the dress fitted, etc and to see old friends... so the size in july will be the most accurate... don't worry about the store policy on reporting defects in sizing.. i will make it work on my own.. please hold onto dress for when i arrive in july"
Note: if the dress is defective now, there's no changing it in july, the manufacturer will not accept it back.
Does anyone truly know someone who can do alterations in under 2 weeks?
What if the dress is too small now?? You can't let a dress out if there isn't enough fabric.
Would she care more if she bought it HERSELF? G bought it for her ($280).
Any suggestions on how to deal with her?
Thanks for listening.