First, the guy that did this should be culpable. Second, whoever let him do it should have dealt with the matter (What if he'd messed-up your boat? Did he have insurance? I'd be chasing the insurance company, etc.)

It's bad enough to do that work outside, but doing it in a tightly packed indoor shed is VERY irresponsible on the part of person that did it and the manager of the facility.

But, in answer to your questions; AND, in furtherance of Larry's observations below...

IF the question is how do you remove the paint, then I'd go for soda blasting and then sealing the concrete with a conventional sealant. The floor should have had a sealer anyway (but it might have worn-off in the course of time.

The other option, IF you prep the floor to make the bonding possible over the ablative paint (which probably would be soda blasting vs sand blasting or abrasive treatment to make the surface bondable. Id' check with the the manufacturer of the paint would have some suggestions), is to put a paint on it that will hold (like a proper garage floor enamel or epoxy).