The CPU part already toggles between 4.1 and 4.3GHz under load so even if he could overclock it would likely not help much.
There's no CPU (or APU) that he can buy that is better for that motherboard. It's already similar to the X4-880K.
Most modern graphics cards are lucky to get a 10% boost with more overclocking.
*Having said that, there are still plenty of games that his computer can run quite well. I had a GTX680 2GB card which would be pretty close to a GTX960 and his CPU while on the lower end relatively is still sufficient to achieve 1080p/60FPS at medium/high in many games.
Games VARY so much though but again lots of great titles that aren't too demanding at settings that still look okay. Especially OLDER games that are still fun to play.
THIS is an example of CPU scaling though it's a better graphics card:
https://www.techspot.com/review/1128-rise-of-the-tomb-raider-benchmarks/page5.html
He sits above the FX-4320 which means in theory the appropriate combination of medium/?? settings could achieve 60FPS much of the time (again I'd force Adaptive VSYNC then tweak to maintain 60FPS say 90% of the time. On 60Hz monitor.)
Summary:
- not really a "horrible" PC IMO. Lots of fun to be had.
- not much "FPS" or general improvement to be had short of spending money
- best choice IMO is to choose games appropriate to the CPU and GPU, then tweak the settings appropriately
Other:
Some games like Starcraft 2, CIV5 may switch from GPU bound to CPU bound as army size etc ramps up