I've just heard that it is bad for the main battery to leave a hybrid (Prius gen 3 2013) for months at a time without driving it. I have a bi-coastal life style these days and am away for 3-6 months at a time. My nephew is living at my place taking care of things and I have him start the car every month but I told him he doesn't need to drive it, just pull it out of the garage and let it run for five minutes. Maybe that's not enough? Does he need to drive the car longer or more often to maintain hybrid battery health? By the way the car has about 80K on it.
From the 2010 owner’s manual: if the vehicle is left parked for a long time the hybrid battery will slowly discharge. For this reason, be sure to drive the vehicle at least once every several months for at least 30 minutes or 10 miles (16 km). If the hybrid battery becomes fully discharged and you are unable to jump-start the vehicle with the 12 volt battery, contact your Toyota dealer. Getting the engine up to operating temperature help in many ways. It needs to be driven.
For just the traction battery charge, until the engine stops by itself is enough. The car is happy with the state of charge at that point.
Running for 5 min every month or so is probably enough for the traction battery...but if he's going to go to all that trouble, might as well drive around the block once or twice too. Mike