Massachusetts primary/secondary insurance question

My disabled teen has a commercial insurance plan with a large deductible and 80/20 coverage as primary and Masshealth as secondary. I signed them up for a treatment with a provider who is listed on the website of both insurance companies. Uploaded both cards into the practice's EHR. Was never told there was any issue with either insurance. Teen was seen for several months by the specialist, then I started getting large bills indicating they had only billed the commercial insurance. I asked them to please bill Masshealth for the remainder. They stated they don't take Masshealth, have never taken Masshealth, and the provider must be showing on the Masshealth website in error. They plan to send me to collections. Who do I contact about this? I know that Masshealth does not do any sort of reimbursement under any circumstances. Had the provider told me they didn't take Masshealth, I would not have kept the appointments, as I can't afford to pay out of pocket.