I'm afraid you are going to have a hard time upgrading that. That is an all-in-one PC, it uses laptop components rather than desktop to keep the low profile. It does actually have a slot for an MXM laptop GPU, but you will have a hard time finding one, they aren't available for retail, you'll have to go digging around ebay or something like that to find one.
As far as power goes, your PSU is an external brick connected to the power cord, similar to a laptop power cord. Odds are you won't be able to upgrade that, or if you can, you'll only be able to do it through HP. These sorts of power supplies tend to be proprietary, and will only work with specific computer models.
I would say it isn't worth even trying to upgrade that machine, even with the lower power requirements, most decent laptop GPUs are probably going to need more power than what the current power supply can deliver. At best, you'll get something a bit better than the integrated graphics, but still not really good enough for any serious gaming.
If he wants serious graphics horsepower, he really should consider getting a real desktop, and not one of those all-in-one systems.