Hard to tell from the photos how bad this is. I assume you are putting tile over the top. The best thing you could do is find an experienced tile installer to take a look at it. Thinset and maybe a grinder may be able to fix this up, but only an experienced tile installer could tell you for sure. If you have waves in the substrate, you will have waves in your finished tile. I have done a few tile jobs in my home and know that tiling over a ridge is bad news.
From the looks of it, I would not let this person in my house to do any more work, 20 years or not. If it needs to be rebuilt, do it yourself or hire a different person.