Unless you have a compressor airbrush (the type used for airbrush makeup, nails etc) I don't think you'd get a controlled enough spray to just cover the white strap marks, it is bound to go onto the surrounding darker skin which will just make that darker.
I haven't ever tried this but if you are confident the tan solution you have is a good match for her existing tan, maybe decant a little and use a triangle cosmetic sponge wedge and gently dab the tan solution to the white strap marks only? My suggestion would then be to see how that develops and then just lightly spray tan her all over (use a darker shade on her legs if that's what she wants). I think it's quite hard to completely even up white marks though. X