I don't know what to advise you, but I'll share a bit of my experience.
By all accounts, my late husband's first marriage was not a good one. He left his first wife after she told him that she'd shared a room with a younger male colleague on a work trip. This was someone who who was at their home on a regular basis and DH had long had his suspicions. (DH had adult kids who had left the family home already.)
2 yrs later - when they'd been separated, DH had bought his own place, etc - his ex offered him the chance to "move back home". This was bizarre - her colleague was her boyfriend by then: they split their time between their two houses. I have no idea what she told him.
They'd sorted out the money themselves without lawyers - the ex earned a bit more than DH by then, so that wasn't an issue. DH made the mistake of agreeing that a friend of his ex's could value the family home. She bought him out and - as DH later found out - only paid him about half of what she should have. If DH had agreed to go back, he'd have been living in a house that no longer belonged to him.
I think what happened was that she'd heard that DH had started to see me. (We weren't in a serious relationship - just coffees and so on. I was fairly shy and naive for my age.)
DH laughed at her and told her that there was no way he was going back.
About 6 yrs later, DH and I got married. The ex and her boyfriend tried to gatecrash our honeymoon. (To be fair, I don't think her boyfriend knew what she was up to, but I'm betting it was deliberate on her part.)
The point I'm making, OP, is that my husband's ex found it difficult to let go even when she had evidently moved on.
Your partner's ex isn't going to manage this in the space of just over a year. =