You can’t.
If you start bugging him to discipline her, eventually he’s going to resent it.
Once she picks up on the fact that he’s disciplining her because you said something she’ll resent you. Add to the mix that she’ll probably tell her mother that your telling her what to do or disciplining her and by the sound of it all hell will break lose.
I’ve been in an almost identical situation as this and believe me, think seriously about making any sort of commitment with this guy unless you’re 100% happy with the way things are going now, because they won’t change.