Maintenance query

I am writing as the fiancée of a divorcee who made no official agreement with his now ex wife when they divorced regarding their children. They have two teenage children and this is the situation. My partner (male) has full custody of the children. He did this as he is in the armed forces and wanted to be able to keep them in the boarding school that his employer is paying most of the fees for. His ex wife wrote a document saying she wasn't in a position to be able to look after them. As far as the employer is concerned, the children do not see their mum and my partner is responsible for the fees. Up until a few months ago my partner and his ex were splitting the fees that were not covered, which is 10% of the fees and quite a significant amount. The children would spend half the holidays with each parent.

A few months ago, my partner's ex suddenly phoned him and said she couldn't afford to pay for her proportion of the fees anymore so he now has to pay double the amount, which is a lot for him. So all she is contributing to her children is any money she spends when they visit her and pocket money. She does not pay any maintenance. My partner is covering the rest as they are at boarding school the rest of the time.

He feels completely helpless because his ex is a volatile character and he worries if he demands maintenance etc that she will react and try to get custody and take them out of the school. His priority is keeping them in this school and so he has just agreed and is now struggling financially.

I would like some advice on a) if she would be able to get custody now following it already being resolved and b) what she should be paying maintenance wise, as it's not as simple when the children are at boarding school.

can somebody help please?

There is no such thing as custody and hasn’t been for years.

Were the arrangements for the children dealt with by a court? Your opening paragraph suggests not but then you mention a document.

How old are they and how long have the current arrangements been in place?

Have her circumstances changed?