On the one hand, yeah, the majority of comments are right in that PS4 will get him a million times more bang for buck than Switch right now, objectively speaking. But if he hasn't gotten a PS4 by now, is it really up his alley? Especially considering he's not much of a console gamer, Switch might be more palatable to him due to the tablet form factor. There's also a lot of multiplatform overlap between PS4 and his gaming PC.
But P5 and Horizon...
I don't know, it's really a tough call. PS4 is the objectively better choice, but I'm not 100% convinced it's right for him. But Switch is still too early on to be a very good value either. I think if I had to choose one, I'd go for Switch though. It's the new hot ticket item and I think he'd appreciate it a lot for that alone "omg how did you find one," etc. Not to mention that it sounds like he'd get more out of Zelda than the other two PS4 games you mentioned if he's into open world RPGs.
I just worry that, not being a console gamer, he'll see this gigantic slab that's gotta get hooked up to his living room, with such a big overlap with his current PC library, and be like "what do I do with this?" But you know your cousin best op. Good luck.