What to do about water in ceiling from leak?
Chances are you can claim from upstairs, if they have insurance you have nothing to loose from getting a professional plasterer to fix it. if they don't and you want to do them a favour then the D.I.Y. approach will save them money. Seeing as you're asking here instead of in &quot;law&quot;
Make a hole, or holes to let the water out.
Wait until it's dry. (possibly weeks)
Patch any weak parts and the holes
Sand flat, seal with a shellac based primer, and repaint.