its an FX CPU so i would recommend a CPU upgrade first. you will have to get a new motherbord for it too if you go that way but sytill a new motherbord a CPU combo will help update your core PC build
that way you won't have to worry about a modern GPU or an SSD issue when it comes to compatibility with old hardware
Why?
The CPU isn't horrible. It can still run games fairly well. The current lead weight on his system is that geriatric 660 Ti. He'd see way more of a boost from a GPU upgrade than a CPU one. The GTX 1650 Super (or RX 580, or 1660, or GTX 1060 6GB) is going to be more than 80% faster. He isn't going to see that kind of boost unless he gets something like a Ryzen 5 3600X. The GPU upgrade will give him a good boost in gaming and hold him over nicely until he can afford a reasonable CPU upgrade.
You know, I've never understood the flak the FX CPUs get. Sure, they aren't as good as Intel CPUs, but they are far from the useless garbage that people have portrayed them. They process well enough, and while a little power hungry and hot, it isn't like they are failures at what they do.
I mean, the FX-8350 is on par with a very low end chip that is on sale right now, it is called the Athlon 200GE, and people get that to play all kinds of games just fine when it is paired with a reasonable GPU. I owned a Phenom II x3, and up until a couple years ago I could still get it to play then modern games acceptably and it was paired with an R9 270X, a decent, now a bit lower end, video card. The FX is way faster than that. Now, I wouldn't recommend buying one today, but if you have one there is nothing wrong with trying to get more out of it till you can upgrade.
Besides, it can handle driving a good 1080p GPU with playable frame rates, so it isn't like the GPU will be useless with the new CPU. Getting the GPU first is a sensible upgrade path. If he upgrades the CPU first he get no gains in games. That 660 Ti is already maxing out. More CPU won't get anything more out of it. Better to get an upgrade where he can see a change.