Just to be absolutely sure he didn't mess with your oil filter, I would open up the oil filter canister and make sure he didn't put one of those small pancake power steering filters in there. Sounds like he was just messing with your power steering pump, but I'm concerned that you said the oil was thick and black.

It should be clear and his putting regular oil in there should not have made it all that black. So not sure what combination of stuff he was messing with, but the power steering pump gets the flat filter and clear power steering fluid, and the rear canister should get a tall filter and that is the one where you should be seeing thick black oil.