To repeat: if the ink has soaked all the way through and the things he already tried haven't worked what you now have is black plastic. Alcohol, napatha, UV, acetone, sand paper, and whatever else are not going to work. The purity of your solvents doesn't matter at all. Anything that you do that manages to get the ink out will probably do more damage -- at least cosmetic if not functional to the plastic. You might be able to get (more of) the top layer clean, but the deeper levels are not coming out. It will just make it look older and more worn.
Just email Fluke and ask them if they sell replacements for your model.