Hi, this sounds bogus. Maybe you talked to someone who doesn't understand the rules completely? Or, if you tried the application online, healthcare.gov sometimes sends you down the wrong path which screws up the subsidy estimate. In that case, just call and talk to someone at healthcare.gov on the phone, and explain your MIL's situation.
Your MIL should get almost the same tax subsidy as last year, if not more. She remains ineligible for Medicaid for her first 5 years on GC (except in CA,) so there should be no change in subsidy/PTC beyond some minimal fluctuation in the insurance premium itself (certainly not zero subsidy.) Please message me if you need help. All the best!