Yeah, he should really turn mitigations on, so that when running arbitrary code from the internet he can be subject to 9999 vulnerabilities, instead of 10,000.
The mitigating factor is actually that you don't go to malicious websites all the time, hopefully. But it happens, including with injected code on ads and stuff that may enabled by secondary vulnerabilities.
[1] Not even including "potentially exploitable from JavaScript", which Spectre was. It's sufficient if you name one where an ordinary userspace program can do it.
The exploit is being able to do it from usermode through an api (browser/js) that normally forbids that.
Userspace can only access its own memory, rather than the whole systems.