The problem with the 7700k is that yes it's amazing for gaming, even beating ryzen. It's the best gaming cpu you can buy today. However, the problem is, it would be not such a good contender for streaming. It all depends on what he does more of. Gaming= i7 7700k. Streaming= r5 1600, r7 1700, or 6800k. Streaming needs those extra cores that the 7700k lacks.
Furthermore, I can provide a better answer for him specifically if he provides information such as gpu, and monitor res./refresh rate.