Potential issues of wet ceiling damage
"It depends"

I own a duplex (upper and lower unit) rental property built in the late 40's early 50's.  The original ceilings were made up of double layer plaster board which is very heavy to begin with, and it was nailed in place.  

A tenant left the water running in the kitchen by mistake in the upstairs kitchen and the drain was blocked.  The water soaked into the ceiling below and caused a substantial part of it to collapse.  Some sections collapsed before others.  One section fell, hit a tenant on the back of the neck/shoulder area, and had to be hospitalized.  While the upstairs tenant was at fault, we got sued, and my insurance eventually paid the medical bills.

In your case, having been built in the 80's, there is less chance of this being a concern.  However, the facts as you've laid them out are:

1) The contractor was negligent with his tools by using them in an area where water drainage could cause damage;
2) The contractor does not wish to fix the issue.

You should insist upon him fixing / replacing it, at his expense entirely, and get a lawyer just in case.