Mousemonkey :
At some point in the near future I shall have a go at running a 7790 and a 660TI in the same PC and if both cards run then a ban may be issued for spreading misinformation.
The Nvidia card should always Idle when the AMD card is installed. So if the OP is testing his friends card on games it will not test effectively.
I got this information from 2 other forums, So I'm pretty sure I am right.