She NEEDS to speak to a solicitor who specialises in family law/divorce.
My sister has recently been in the same situation where her husband was insisting on 50/50 custody so that he “wouldn’t have to pay her a penny”.
He then found out that she was entitled to a higher percentage of their combined assets (mainly their lovely 4-bedroom home) because she earns less (she works part-time due to health issues from the trauma her ex has put her through and can’t manage more hours, but she works in a high stress, high responsibility position). She’s also entitled to some of his pension.
Funnily enough, once he knew that he agreed to a 60/40 split - but did keep pressuring her with lines such as, “if you agree not to go after my pension, I’ll agree to pay child support….” But she had a good solicitor who kept her well informed as to what she was legally entitled to and stuck to her guns.
He was abusive (not physically to her, but verbally, financially etc, did get physical with the kids, intimidated her, bullied her, never lifted a finger to help with either of the kids, cheated on her, etc) so she owed him nothing. He’s an awful person.
Your sister really does need to speak to a solicitor so you know exactly what position she’s in and what she’s entitled to. She will need evidence of joint assets (property, savings etc) and earnings, and if she can do any digging to see if he has anything set up in secret, that will help her too. My sister found multiple secret savings accounts that her ex had, which he’d squirrelled money into from their JOINT savings accounts (that he denied her access to).
Good luck!