Unfortunately it is an illegal copy yeah. The only way anyone can possibly legally get a copy of Enterprise is through Microsoft's "Home Use Program" which would be provided by a workplace for employee use at home. Even if he did obtain the copy this way, he cannot legally pass it on to anyone else.
To me the biggest limitation there is probably the graphics card, the CPU is also pretty weak but should be fine for current games. If you are looking to upgrade your graphics card, you'll need to take a look and check that you have a decent quality power supply - Due to the other issues with the PC, I imagine it's probably a pretty cheap one you may want to replace if you are getting a better graphics card.
That said, I would first take the PC back and demand a refund instead of asking for a legitimate licence - He'd probably rather do that and then sell the machine to someone else instead of paying the large amount of money for a legitimate licence. You could then get a new PC that's better for gaming from a more reputable source.
