I've been tanning for a few weeks now and get a tan every Sunday. This is me trying to promote my spray tanning!
As someone said above the best way is to have a good soak in the bath and get some exfoliating mitts on and scrub with some shower gel or something. It will come off. Some areas are more stubborn than others, one especially is around the arm pits! I personally do not spray there too much as I know this is a problem area, but my husband who sprays me doesn't take a telling!
But as long as your client nearly gets it all off, it'll still look good!