Melanin only tans the top layer of skin so when you wax and remove that layer, off comes the tan - regardless of if it's a spray tan or a sunbed tan or a natural tan.
The only layer of skin that will change colour is the top layer and that gets taken away when you wax so the tan is taken with the skin.
She may have the slight tanning effect continue from her last bed or 2 show through a day or 2 later as you say but if she has been going on the beds for weeks in an effort to get a tan to go away then waxing is going to remove that tan and IF she is lucky she will get the little mealine reaction happen after the waxing to give her a little of that colour back.