If you file Married Filing Separately, that disqualifies them from receiving the premium tax credit. So they will have to pay back the Premium tax credit on their tax return.
There are exceptions for certain victims of domestic abuse and spousal abandonment. For more information about these exceptions, see the Premium Tax Credit questions and answers.
For more information on the Premium Tax Credit, go here: The Premium Tax Credit- The Basics
[edited 02/13/23|8:47 am PST]