Which means they are covered by it.
Only if they leave the country, which is voluntary.
It is ALREADY customary that Green Card holders that re-enter are subject to review:
What documents do I need to present to reenter the United States?
If seeking to enter the United States after temporary travel abroad, you will need to present a valid, unexpired “green card” (Form I-551, Permanent Resident Card). When arriving at a port of entry, a U.S. Customs and Border Protection Officer will review your permanent resident card and any other identity documents you present, such as a passport, foreign national I.D. card or U.S. Driver’s License, and determine if you can enter the United States. For information pertaining to entry into the United States, see U.S. Customs and Border Protection’s webpage.
So basically, they can leave if they want, but the have been declared as inadmissible if they return prior the 90 day wait and MAY have to be re-screened thereafter.