I don’t think he needs to stop seeing his daughter or that you should not let him see her. And I don’t believe you even suggested that.

But there is no rule that one must see their poorly behaved adult kids in the house (whose house is it btw? You two are dating so it’s fair to assume you don’t live together?). People see their adult children in variety of places: restaurants, shops, outdoor places, parks, in kids’ houses etc Even during pandemics there are places where people can meet if they so desire. Now if she was a minor of course I’d say she needs to be in the house with her dad. It’s no matter if children behave poorly, they belong at home with parents. Not adults, unless they are incapacitated.

I disagree that the only way for the father to see his grown daughter it’s for her to come to the house and a girlfriend stay quiet. There are plenty of ways for people to maintain relationship.

I’d say if a boyfriend is too lazy or passive or clueless to find ways to maintain a relationship with his daughter and do not allow disrespect towards his girlfriend at the same time, I’d say he might not be a boyfriend worth to keep. And it might be off topic but him still being just a boyfriend after SIX years might be a symptom of something too