If they want fair market value - advertise it on the internet and take it to auction. Check out the Burwell, Nebraska '73 911S thread (
http://forums.pelicanparts.com/porsche-marketplace-discussion/675203-dellusional-prices.html) to see how that played out. Widow put a reserve at $27,5k but the car sold at $92k because it was advertised and auctioned appropriately. The family received what the car was worth (less ~10% auction house fee) in the end.
Buying this 356 from them to flip it isn't doing the owner any favors. You may profit (and if you take the risk and are able to buy it, good for you) from a transaction or end up with a cool car for yourself.
Kudos to you and your friend for finding what sounds like an original 356-C coupe. Cool cars and worth a considerable amount depending on condition. No doubt the family understands it is worth something but they may not know the present state of the market.