Your friend isn’t unreasonable in wanting financial security. She is however unrealistic in thinking that she will ever find it with someone like her partner.
If she tries to stay and fix him/the situation it could get much worse.
I base my opinion on my own experience as a child in a similar situation.
My mum did similar with my dad over gambling. Took over the finances and got the family situation in a much better place pretty quickly.
He was still an addict though and so he just ramped up his activity with other women to “compensate”.
When my mum asked him about that, he punched her so hard he cracked one of her molars. She picked me up out of bed in the middle of the night and ran.
We went a few streets away, to my nursery (two nice older sisters ran one from home on the ground floor of their house).
My mum blamed herself for years for “making him hate her and never forgive her” for being to handle money better than him. It totally wasn’t her fault but she was too traumatized to hear that for a long, long time.
She only really got over it when she was terminally ill and reflecting her life.
It also came out during their decade long hellscape of a divorce that hat it hadn’t totally stopped him either. He took out loan shark loans and got into debt with illegal bookies/went to illicit gambling clubs and card games.
He also committed fraud creating liabilities and claims over shared property. It took nearly another after they were divorced to sort out the financial and legal mess.
My mum had nearly twenty years of legal battles from the fallout. Some directly with him, some with banks and financial institutions looking to recover their money. It broke her mental health for a good part of that time.
He conveniently went bankrupt very quickly which at the time meant he didn’t have to pay child support. My mum and I were also made homeless at one point.
My mum had to fight the bank over repossession of our homeas my dad had fraudulently used it as security for something else.
It wasn’t even the marital home it was a smaller flat that was as bought out of the proceeds of selling that as part of the division of assets during the divorce. Part of the reason my mum had to fight that was so she wasn’t seen as making herself voluntarily homeless if we needed social housing. Really it was an endless nightmare.
If he is as angry as he sounds, he could be a danger in many ways. Physical, emotional, financial. My dad tried to get my mum committed at one point.
Be aware that if your friends partner is throwing out accusations like financial
abuse it could escalate further. My dad tried to report my mum’s GP for sleeping with patients (my mum) because the GP intervened when he tried to have my mum committed. He told the local mental hospital it was nonsense, his patient was mentally sound but her husband wasn’t and he was dangerous. The complaint went nowhere but it was a stressful and horrible time for everyone involved.
Really, honestly, your friend needs to get out fast.