maxalge :
bystander :
How much GPU you need is determined by your resolution as well as the target FPS you have in mind. The only thing the CPU is going to do is limit how high of FPS you can maintain, but as long as we are talking about 60 FPS, your resolution is what matters.
What's your resolution?
cpu has a direct effect on fps - lows, averages, and highs
The CPU effects CPU bound areas of a game. But resolution you play it, determines how much graphics power you need for what ever settings you are after. If 60 FPS is all you are after, his CPU will get him there in most cases, with a few lows here and there (I had an i7 920 only a few months ago).
If he's after no more than 60 FPS, the resolution is all that really matters in this choice. If he's after 100+ FPS, then he simply can't get there with his CPU in many games, but I'll still need to know the resolution for his choice.
If he's using a 1080p monitor, a 1060 is the max he should go for. For 720p, then a 1050 would be enough. For 1440p, a 1070p may be worth using. For 4k, at least a 1080 would be good.
That would be the same for any CPU running at 60hz.
Edit: and based on the budget, he probably can buy the fastest GPU he can afford, unless we are talking about a very low resolution.