If this tub is a shower/tub, then I would say the painter is wrong. If its just a tub with high tile walls, then there is not a problem.
I am assuming and speculating that the tub is shower equiped. I would not do this for you if you wanted a water proof wall. I would not want to tell you something untrue them be liable for it when the water deteriorates the wall. Therefore, if this is to be used as a shower, I would take the tile all the way up and forget how much you love the plaster walls.