She can leave anytime no problem.
But coming back could be a problem. Technically, as a PR, you need to have your PR card to board an airline. It can take 60+ days to get your PR card once landing (they will not mail it to an address outside Canada).
However, since she is from a visa-exempt country she should be able to board with just her Korean passport and at customs in Canada they can look up her PR status in the system and let her in.
The only other issue I can think of to be aware of is if she is in Ontario, she needs to spend 153 of her first 183 days in Ontario to qualify and keep OHIP. If she goes to Korea and then stays for over a month, then obviously she cannot fulfill that requirement. I don't think the other provinces have such a strict rule.