My partners ex won’t let him see his kids

Hello. I am wondering how you Mama’s would deal with this situation. My partners ex wife is not allowing him to see his children for no reason - besides spite. There is no parenting plan in place as of yet and she is beyond impossible to reason with.

I guess my question is: how would you handle this, more so from his perspective than mine. Any advice is appreciated as this has been a very difficult battle.