Depending on the airflow through the case, it should work
 
900 series are Not UEFI bios required cards  (I've run SLI EVGA 980 SC on a X58 chipset MB)
 
DDU is a very good way to remove old GPU drivers before installing the new driver. (use Safe mode with the reboot after changing cards)
 
ATI/AMD cards can require a little more work to remove those drivers before switching to an Nvidia based GPU.
 How-To Uninstall AMD Catalyst™ Drivers From A Windows® Based System
AMD Clean Uninstall Utility
 
If the PSU is not single rail design, you'll have to balance the load
 
If you do have an issue with the GPU not being recognized, after confirming ALL connections, then:
 
You may need to record all BIOS settings (camera on your phone is quick) and then clear the CMOS and reset BIOS to factory default settings, reboot, enter BIOS and place your settings back as they were ---> this allows the CMOS to record the new hardware

You also may want to check how old that psu is.
If its 5 years old, that psu may be down to only outputting 2/3 of what it did originally.
I hope you get everything sorted out, and everyone is happy.
 
I had my buddy give his dad a 290, and his dad had an older hp machine. Well, it ended up frying the video card and the psu, and he ended up having to buy all new hardware, mobo/ram/cpu/video card. Just because of an old psu.
 
So, if its an older psu, just have him get a newer one with the card.