Skimmed ceiling and wall - plaster cracked and falling away

You need to give a few more details in regard to background. For example, was he skimming over new plasterboard, old lime plaster, artex etc. It could be the background wasn't sealed adequately, or parts of the existing plaster were live, or blown. It could be out of date finish, or dusty or dirty boards, or it dried out too fast because it was hot and he had the window open with a breeze blowing through. If artex it could be it's shelled because there was no suction and a bonding agent should have been used. With the cracks you mention, I'm guessing it's over old plaster and there's way too much suction because he didn't seal it properly. The whole lot will probably be gone.