How to get out of a mortgage with a belligerent ex?

My partner and I are trying to fix his credit score. One thing impacting it is a mortgage he entered with an ex (to whom he was never married) who has become belligerent since the break-up. How can we force her to let him out of it? He doesn't live in the same state anymore. He doesn't contribute to the mortgage at all. She bought the house just to rent it out, so her tenants cover the mortgage, and she keeps the excess as supplementary income. I've seen that his ex would need to refinance the mortgage in her own name, but she is a notably vindictive and spiteful person. It's really amazing. I'm positive that she won't willingly help us. Is there anything we can do?

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