Babying ruins as many cars as wringing them out occasionally does. A car that's been in sitting the garage for 5 and a half years is just waiting to have problems. And who knows, your friend may not have broken the engine in correctly, or there may be a fault that nobody is aware of.
Get the PPI, it's not a matter of trust, it's liability because you don't want the friendship to suffer if you both learn something you weren't expecting.
For the record, I got a PPI and appreciated knowing what expenses were around the corner (CV boots, water pump under warranty). Also get a compression check if you want to feel good about the engine, I got 150 across the board and knew I had a winner
-David N.