Problem with the ex wife :-(

I am dating my boyfriend since 6 months and at the beginning i had no problem with the fact that he was married and had a good relationship with his ex wife
The more we dated, the more I discovered about this relationship and the more it bothered me
The ex stole all of his money and insists on going back with him. She asks him to go shopping for her, to repair everything in his flat and to lend her money constantly. He was still paying visit to her in her flat at night. She is not informed that he has a new gf.
I asked him if he was willing to stop seeing her because i feel uncomfortable with this situation and he said yes, that he will not see her anymore or have contact.
But then he asked me if he could go for lunch with his ex wife.
I do not understand why he needs to see her and what does he expect from this relationship.
I know we cannot change people behaviors but this relationship really threaten me and i do not feel safe or respected.
It feels like we are in a conundrum because he keeps on explaining that she means nothing to him and i keep on saying if she means nothing then why keep on seeing her ? I tried to change my thoughts around it but i cannot 🙁
Any tips ?