I'm slightly confused here....if she got the strap marks after you originally sprayed her - why is it your responsibility to put it right? Is it not her fault? Or is she simply looking for your help....(paid I hope)!
I have from time to time, 'filled in' white bits for clients, and find what is easist to do is lightly spray the lighter area, and then once its dry, spray the whole body, but I wouldn't suggest she has a spraytan on top of recently burnt skin at all. The skin may a/ react differently. and b/ if dry from the sunburn could grab colour making it look worse.
Personally, I'd strongly advise her to wait util the whole area calms down and then you can see what your dealing with. If the lines aren't too obvious an all over tan would probably even them out quite considerably, but if there is a quite a colour difference then you can tell her you can attempt to even it out the way I suggested above, but warn her there is no guarantee of perfection on unevenly toned skin.