The thing to do is just to be brave. Decide that you can replace the zip whilst hoping it has buttons 😊
Get her to wear it inside out and pin, I use safety pins, making a fold / pleat that makes the body the right length.
Inside out you can see any pockets and bits you actually need. You can also see existing seams.
Finding the existing waistline and shortening there, lifting the legs up into the body, should leave room further up to accommodate her boobs. Obviously this means you shorten the body before you shorten the legs.
Taking it up into the sleeves also becomes possible if you can see the seams.
Basically, put a pleat in where you think you can take length out. This way you keep any of the tailoring that already exists.
Then simply sew along the bottom of the pleat, turn it the right way round, try it on and see how it feels to move around in. Once it feels and looks right double stitch the new seam and cut off the excess material.
If you are lucky you can butcher the bottom of the zip by sewing a new bottom stop bar (just see over the teeth a few times and maybe add a stitch to the sides to make it tidier) at the new seam you are putting in and cutting off the bottom of the zip too - do that with the zip fastened.
Buy two and be prepared to mangle one.
If that's too expensive by a dress or jumpsuit from a charity shop and do the same thing, get used to the pleating and cutting on that!
Just be brave and have at it. Best if luck!