For the situation stated above, it would cause and amount to show up on form 8962, page 1 line 11, column F and cause the diagnostic. This diagnostic does not stop the return from being e-filed. I do not know if they would have to pay the amount,  I would suggest taking a look at Form 8962 instructions to see what they say. I am not sure why they would have a 1095-A if they were not enrolled in the marketplace.