Even though the Underworld was arguably already ruined? Also, my interest in this isn't actually about the Underworld itself. I'm treating that more as a piece of the larger puzzle. If the caretaker was correct in her claim about what he is, I imagine that could have broader implications beyond just the Underworld. Willow could also take advantage of his access, theoretically, to have him retrieve things for her or to try to get information from the well.
There are other ways to approach it that aren't him relying on her to fix a problem, and that don't require her to be able to do anything about the Underworld.