Leaky S you had no obligation to tell the new owner or his dad anything about the IMS.
You chose to because they are friends of yours. Theoretically a person should get a PPI done on a used car before purchasing. Buyer beware etc. But many buyers don't.
The best advice I would give is to suggest getting a new IMS installed. However owning a Porsche there are other preventive repairs that could be made in addition. A new water pump? He may find in time that Porsche ownership is not for him. It can get expensive. But that is his choice.
