A nitpick here, but "visa" gets tossed around a lot. A visa is a document which allows you to request permission to enter a country for some purpose. To work in a country you need a work and/or residency permit. Even in the US, there are situations where you can work with a an expired visa, provided your other immigration documents are current. But if you leave, you need a new one. Similarly UK citizens can still work in some European countries without a visa, because there is no requirement for them to enter with one (but the officer at the border might still refuse entry).

You can be grandfathered in, but since this was about somebody returning to Europe, it likely doesn't apply to them