She can NOT apply for a visitor "visa", as UK citizens are visa-exempt. She only get visitor "status" when coming to Canada. And since it's been 90 days since her "status" expired, she can no longer request to restore that status.
As I said, the only way for her to get legal status again is to leave Canada and then re-enter. If she re-enters successfully that would automatically give her new "status" in Canada, and she could immediately apply for her OWP.
It's too bad you didn't do an outland app as she would probably already have PR by now or close to it, but that is water under the bridge. Research the risks involved in leaving/re-entering Canada, and see if it's something you would want to try to speed up her OWP. In showing a PR app in progress, traveling with you, and able to explain previous overstay if asked... risks of being denied entry by CBSA should be small, but of course not guaranteed.