The Pearson 26' is what you are calling the Commander, I believe: PEARSON 26 sailboat specifications and details on sailboatdata.com
5000 psi for a power sprayer is pretty powerful. I believe I used a 3000 psi sprayer on the hull of my 1967 Tartan 27' (cousin of your boat) and it did gouge the older layers of ablative paints off our hull in a few spots. I was doing the spraying though.
Without pictures of the damage you are asking about it will be difficult at best to fully understand the nature of it.
I'm not even sure that Pearson used gel coat on their hulls back in '67. I'm fairly sure I don't see gel coat on the hull of my boat of the same vintage; this being 45 years after it was made. I know that there are some spots on my hull that may have been repaired in the past and often require a quick patch now and then.
It is possible that a high pressure sprayer like that may have dislodged some bits of your polyester resin based hull. What does this tell you? It might tell you that the old polyester resin can crack or abrade under the right conditions. It might also tell you that you should think about filling and fairing these areas and consider taking off all the bottom paint and applying a barrier coat to the hull.
So who's at fault?
For starters, you are. You own a 45 year old boat (as do I) that is no longer under any warrantee; with boats of this age more stuff is at, or past it's expected lifetime limit and will need fixing, or replacing sooner than a boat that is 20 years old say.
Perhaps your boatyard is to some degree but you had better check your contract with them. Perhaps they would give you a good deal on the job of fixing it. Perhaps not.
You might be able get your insurance to kick in some for the damages but good luck with that. An estimate of $10,000 to really redo the hull would likely cause them to total your boat. The cost for doing some grinding, sanding, epoxy/cloth patches should not cost you more than $500 in materials, barely a boat buck.
Pictures would help.