The father of my partner’s children treats her badly

Hi all, I have been in a relationship with my girlfriend for two years. My partner has two daughters but the father of her children wasn’t very nice in their relationship and after splitting up now uses manipulative tactics eg. he expressed a desire to have his kids 50-50, so as to stop paying child maintenance, and even went as far as to try and obtain half of the benefits that she gets for being a single parent so that he wouldn’t have to work and could financially gain from looking after his kids more. When he found out that he wasn’t able to financially gain in the way that he thought he withdrew his offer of looking after them 50-50, and took voluntary redundancy from his job which meant that he no longer had to pay child maintenance. This put my partner at risk at losing her home.

On the days that he does have them he brings them round to her house at 7 o’clock in the morning. Sends the youngest upstairs to have her mum dress her and do her hair and expects the mum to make packed lunches for both of them. This might seem quite trivial, but when she’s looking after them six days a week and working full-time.. but that means that she essentially doesn’t get any time off!!!

She is scared of addressing the situation, for fear that she will lose all his support but I am finding it really difficult to allow a man to manipulate her into a very unfair financial and childcare situation.

I just don’t know what to do and I fear that this ongoing issue is driving us apart and she’ll end up alone under his control with minimal financial support minimal childcare support.

Any thoughts would be much appreciated ❤️

TL;DR - is it my place to interfere in a co-parenting relationship that affects the mother and the children