What is her long term plan? When will she be looking to move out? Has she even looked at the cost of housing, how much mortgage she will be able to obtain, renting costs? This is a much needed conversation.
Your Dh is failing her. Ds1 is 23, has been at home for almost 2 years post uni and knows that his salary only gets him X so the way to get a better or bigger house is to have a massive deposit.
He doesn't pay any money to us but he has a LISA, ISAs and savings accounts with tens of thousands in them. His goal is to move out in 2 years or less. He also does chores as a member of this household like unpack the dishwasher, does his own laundry, cooks family meals and puts the bins out.
Your Dh isn't teaching her to budget her money. It is pretty much guaranteed that when she moves out she will not have all of her wages to spend how she pleases. She will have bills and responsibilities. It is usually these children that move out, end up in arrears, move back home and then find it harder to leave again as they have shafted their credit score.
She is using his kindness, he is a sucker. She should be paying something as this isn't her parents' house, she has a step parent who shouldn't be covering her financially. It also does not set a great example for the other child in the house.