You would brush solution into the "white bits" and leave to develop then next day give her a full body tan. There would be virtually no visible lines, and on the full tan brush in over the lines if there are any.
I don’t think it will ever be perfect but I did it once for a bride with huge thick strap marks in a strapless dress. I did it as normal, as far as I could tell she looked good in the photos. If it was severely obvious then you could put a bit of barrier on any very tanned bits but instead of fussing about, just give them very low expectations. It might cover it a bit. Would rather make them certain it may all go to pot and then be a bit pleased you know ?