Transferring domain name from unwilling "owner"

I have a domain issue I am not quite sure what to do with, and I hope someone can share some insights.

My girlfriend has an online fitness studio, with active paying members, and things are going well. However, the domain name and hosting have been setup by her ex (who is a completely narcissist but that might not be the point), and their separation was not very friendly. It’s been over a year that they separated and now we are at the point that we want/need to revamp the website as it’s completely out of date with the theme used integrated into the site to an extent that we cannot update anything without breaking everything. Also, considering there are paying members we cannot really risk anything.

A whole overhaul is needed. However, the site and domain is technically on her ex’s hosting despite him having absolutely nothing to do with this fitness studio. She asked for the domain to be transferred to her (by my advice) but this has lead to huge fights and is not worth the effort.

Ideally we want the domain transferred but without his help I realise this will be impossible. The domain name is aged and perfect but I fear there is no option but to start again. We do have admin rights on the Wordpress site itself, so we could potentially redirect to a new site, but it is all less then idea. What are the options here? Is it possible to claim domain ownership some other way?

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This business is 100% built up by her and it feels like she will be screwed no matter what, probably losing all active members and having to start from scratch.