I misunderstood but none-the-less there are billing companies who are Covered Entities, most however are Business Associates. Can't really help with that one without knowing how they are structured. Then there is an issue of what the contract states. It might be perfectly legal for them to cut off access. I know that If I terminate a provider, a part of my own compliance plan and contract states that access to records is immediately revoked. If they need reports, etc I will provide what I can and adhere to my contract.
If a contractual relationship ends it stands to reason access to the billing companies system would be revoked, but again it really depends on the contract.