and throw in a 750w psu so he can xfire later
I don't know if he will do crossfire or not. He is still working off of like a 5 year old PC now which is why he is using a laptop to play.
would there be an advantages to doing so with such a powerful single GPU or would it be bottlenecked?
Crossfire would be if you want to go 1440p and still play BF3 on ultra or something.
At 1080p that card will run everything you play. You could also just get one 7970 for a little more and it would last you for a very long time.