if you're looking for something with the most security, then Qubes. It's heavy, it's slow, but good luck to anyone looking to break into that system.
Bit of a learning curve and a bit to wrap your head around it but I would tell him to think of it like you have access to a bunch of individual computers that don't talk to each other but you control all of them. So he could have a Qube for casual web browsing, could have a Qube for work, and another Qube for financial stuff. all independent of each other. IF something were to happen (malware, trojan, whatever) just simply close that qube window and spin up another.