One thing you could do is checking to see if there are any BIOS updates for that motherboard. There is a slim chance that could resolve some of the card compatibility issues you're having. But the 750ti might be about the best card to use in an older system.
After thinking about it for a while.. I realized one of my older computers had the same q6600 Core 2 Quad CPU thing inside. At the time I had the 8800GTS-512 card, which ran perfectly and never gave me any problems. Then I decided to upgrade to an HD6850... and that system simply REFUSED to accept that card. I tried everything... BIOS updates.. even contacted XFX and they sent me three separate cards... but none of them worked... and after further research I found out that some older motherboards with NVidia chipsets, like what was inside my computer, would not run AMD Cards at all... It was at that point I decided to just go ahead and build my first all new system.
Another option you could consider is building him a new system, too. You could transfer over as many as the parts as you could... but get a slightly more modern motherboard... Just a thought.