The hulls will flex considerably by hand, hard to describe how much though. I would do a coin tap test to compare the sound of the glass in the bows compared to the rest of the boat.

The 5.2 is not a very valuable boat, especially one of this age. Ive seen newer complete and ready to sail boats trade in the $700-1,500 range. This boat likely needs a few hundred dollars worth of running rigging, re-stiching on the tramp, trailer tires, plus all of the labor to clean it up. I would value it at ~$0. (you might even do him a favor hauling it away for free)

Its not a project I think would be worth taking on unless you really want a 5.2, or you have more time than money and really want a project.

I purchased a 1983 5.2 in similar condition from a sailing/rowing club 100 miles away from home. It was described to me as an 18' Nacra and was $600 so I bought it sight unseen and paid for it over the phone. After driving a couple hours after work to go pick it up I discovered it was a 17' 5.2 and not a 5.5 as the description led me to believe. I decided to let them keep the money and the boat and leave empty handed. I have no regrets at all about that decision.