You do realize a negative 5 is simply reversed polarity. Switching the two pins that complete the circuit around will fix that. It is a DC circuit. If you have 5 volts, switch the red and black around and you have -5 volts. Anyone with basic DC skills can fix that. Where I work we deal with DC voltage all day, every day. It really is that simple, swap the hot and ground wires, voila, -5 volts if he needs it. My bad, like so many others here, I have a tendency to think that because I understand how something works, everyone else should also. That is why I try to go in-depth when explaining something to people, because often they do not, the same as I don't know everything they know. So for those who do not understand DC circuits, if you need negative voltage, simply swap the red and black wire (usually red for hot, sometimes it can be yellow or some other warm color) and you will have negative voltage.
Don't believe me? Take a multimeter and check a battery. Now, try it with the black lead on the postitive and the red lead on the negative and see what it reads. If you have a digital meter it will show negative.
That being said, he has a PCIe port, which has only been out for 13 years. If they haven't really used the negative 5 for ten years then chances are he will not have an issue. If his PC is over 10 years old, then the GPU is the least of his problems. You said his MB was old. I have looked and don't even see where he mentioned what MB he had.
Sure, the PSU is a POS. That is why it has 176 reviews and average 4 out of 5 stars by people actually using it, LOL.