Teenage bridesmaid dress!

If it's just having the waist taken in, it shouldn't be hard to find someone local to do it. If you already know there is a dress that fits apart from that, you could even message potential tailors to ask if this is something they could do in the time available. That way, you know what's possible before buying the dress.

I have used JJs house and they are good, but not sure on your timings if you will have enough time to get it and if it isn't right again you have less time to find something. Have you looked at an adult sizing, as the bust area will likely be bigger than a teens one, so could she get a 6/8 to fit waist? Or the other option is to get it altered, we are doing that for one of my BM's as she has huge chest but is also only 5ft2 and has a tiny waist. I got the Mischa dress from Monsoon as the cut is good for smaller waists and the cups are quite roomy

Depends on what sort of thing you like, but Cicinia have consistently good reviews, and they will make dresses to measure at no additional cost. They also have a huge range of colours and styles, so this is probably what I'm going to go for with my bridesmaids!