Price per dollar, a RX 570 would be best, but if you don't know or can't trust the PSU as much, go with a slot powered GTX 1050Ti. Either one will be more than the CPU you have in there is able to handle, slight bottleneck on the 1050Ti and more on the RX570.
I'd likely recommend this one if buying today, since it's only $100, comes from the manufacturer with a warranty.
https://www.evga.com/products/product.aspx?pn=04G-P4-6255-RX
I'm more on the idea of recommending the 1050Ti in your case, since it sounds like you and your friend have limited knowledge on computer internals and the power configuration makes it the easiest just to drop in without caring about the power supply, etc...