The problem with this situation is he's not coming from a place of genuine care for this girl. He's just annoyed she's costing him money. And both his wife and this girl are likely to sense that and team up against him if he starts pushing harder. Which is not going to be a good thing for his marriage. His marriage, the most important thing here.The way I see it is that having adult stepchildren in the house isn't that different from having adult biological children in the house. You want to support and encourage them to become more independent, and eventually move out. BOTH parents have to be on the same page though, which involves difficult discussions when one parent wants to coddle the adult children. When they are biological children, it is very likely that the mother will be the coddler. When they are step children, it's very likely that the biological parent will be the coddler.
Just treat them like adults. If they are employed, start transferring responsibility for their own bills to them, like cell phone, clothing, toiletries and snacks/treats. If those things are on the shopping list, don't buy them. If the coddler wants to buy them extra things, it should come out of HER money. For home things like utilities and food, tell them these are the bills, your share is 25%, which is $$. Please pay us monthly. If the coddler wants to pay that for them, it should come out of HER money. If issues of payment develop, well, you can discontinue the unpaid cell phone from the family plan. Change the password to the wifi and Netflix until you get their share of those bills. Etc.
You have to make being a freeloader unattractive if you want them to change.
If he can't convince his wife to get on board with steps that might be better for her daughter then he needs to just learn to either accept it or divorce her over it. A pissed off middle ground, constantly trying to save money by evicting her while uttering the lie he's only doing it because it's good for her probably won't do him much good.