The client should contact the Marketplace to verify that.  If the client is young, they should check if their parents signed your client up for insurance.  If your client has any dependents, your client should verify nobody signed the kid up for insurance (such as an ex).

If all of those end up as no insurance, write up an explanation with that information and convert it to a PDF.  Then attach that PDF as an "other" attachment and call it "ACA Explanation".