I misread a few of the early posts and hadn't realised you were talking about a 2600 non-K as opposed the 2600/2700(k) (the 2600K and 2700K are basically identical, bar marginally better binning on the 2700K which is why I used that as an example), apologies! This being said the principles are still the same, you CAN still BCLK overclock the 2600, albeit the results will be more limited. I also didn't JUST talk about the 2700K.
If you can get your chip to 4GHz+, it will still produce a very usable experience for Fortnite on performance mode, albeit the bottleneck will be harder than the 2600/2700K could produce, so really what you can achieve here will come down to your motherboard, CPU and cooling (and how much testing time you have to find a stability point). You may also be able to find a 2600k/2700k or Ivybridge equivalent for peanuts (as most Sandybridge motherboards will take Ivybridge with a BIOS upgrade), all of which overclocked will give you some extra headroom over the 2600 stuck with BCLK overclocking (as that has harder caps on the overclock possible before other elements begin flaking out).
The additional point I raise about newer CPUs vs the older architectures is still completely valid however, a 2600 will still get curb stomped by virtually anything from the last 2-3 years, so bear that in mind, both in terms of performance, and in terms of power efficiency, so it really depends on if your family would throw any money your way to help on that side, BUT, all this being said, your kit will very likely be a decent step up from the old AMD APU. The earlier ones of those were very slow, and I'm guessing it could be something like and old FM2+/AM3 board. Sandybridge was massive jump at the time due to both IPC and clocks, and the early AMD APUs were very performance limited so assuming it is similar era it will get curb stomped, albeit without knowing more about which one they have, its harder to guage just how much performance will be gained.
IF your family will lob money your way to upgrade the machine though, it might be worth seeing if you could get a second hand B450/10th-12th gen package to just go with, as that will annihilate the Sandy and also enable the machine to absorb any performance nuking patches that may come to Fortnite in future, it is definately a heavier game now than it was at launch, and performance mode has just helped keep the barrier to entry lower
Your kit is already there, but it is definately reaching limits, and if they care at all about performance in the future on games other than Fortnite or Fortnite if it gets any future big updates, it's worth considering.
Also worth checking the machine actually has an SSD. If it's still running HDD, then throwing a cheap SATA SSD (even a cheapy chinese no-dram cache) in there will also make a massive improvement to the machine, even for games like Fortnite, as it loads areas of the map in as you move around!
