I do think that people can change (my partner did!), especially with education. But since her parents are frugal it sounds like she has had some education, but still wants to live a flashier lifestyle, then I'm not sure there is much you can do.
I will say that people's spending habits often reflect their values, fears, emotions, etc. If she sees being an adult as getting the things she wants it may be a reaction to her feelings growing up in a frugal home of not having what the cool kids had. That kind of stuff is deep-seated and difficult to get over, but might be worth a conversation so you can understand where she's coming from.
This was actually how I got my partner to change. He was always gainfully employed, and had high earning potential, but he would just buy things and fancy foods he didn't really need without thinking about it, since he could always turn on the money faucet and make more. He wasn't in debt, but when he went to grad school his spending habits began eating away at his savings. When I first brought up budgeting and Mustachianism (about 6 months before we got married, when we were merging our finances) he was resistant. It was really helpful to talk about our feelings around money. For example, I value security, and budgeting makes me feel secure. He values freedom, and he thought budgeting would restrict his freedom. We also talked about our parent's approach to money, what worked and what didn't (actually, I think this is how the conversation got started, because it's easier to talk about other people than ourselves), and what kind of retirement and other long term dreams we want to achieve for ourselves. We also talked a lot about what makes us happiest, and how we can better use our money to facilitate that, in both the short and long term. He agreed to try budgeting (and we had a small amount of no-questions-asked spending money for each of us built into the budget so he wouldn't feel too restricted), and he actually really liked it. I think he enjoyed feeling like things were on track, and that we had a plan and were on the same page, and he realized he didn't feel deprived or restricted in the way he thought he might. We also learned to communicate about money in a healthy way, which has been huge.
So that's my advice. Try and understand what's behind her spending habits, and communicate the thoughts and values behind yours in a way that she can understand (don't just say "my way is obviously better because look at this spreadsheet"). Best of luck.