And I know it makes sense to her. It just breaks my heart to watch it. I know you're all right. It just really is like watching someone walk into traffic over and over. They may have a reason that makes sense to them. But it makes me sad and confused.
Well that's a little condescending, isn't it? Why is your chosen lifestyle "right" and hers "wrong"? She's not dropping heroin or prostituting herself for money. She's just spending more money than you think she should on stuff that you don't think she needs. That doesn't make her wrong, just different than you. I guarantee you that she is looking at your life and thinking exactly the same thing -- "wow, I feel bad for her, so much money, and she can't even loosen up and enjoy life enough to buy a single lipstick."
One of the things that bugs me about financial advice is that thrift is viewed as a sign of good character, whereas immediate gratification is viewed as a moral deficit. Think of the original marshmallow study: kids who don't have the internal fortitute to wait for a second marshmallow end up being the same kids who are poor and needy their whole lives, because that same lack of character leads to many other bad choices. Ergo, poverty is driven by stupidity and lack of character.
Now look at the revisited marshmallow study: the poor kids grow up in an environment where everything is ephemeral, and any good thing disappears quickly. If someone gives you a lollipop, you're not going to stash it in your room and save it for later, because the odds are extremely high that a sibling or friend is going to find it and eat it before you can. From that context, if someone offers you one marshmallow now or two later, it makes every bit of sense to take the one now. Because you know from hard-won experience that the vast majority of the time, those two marshmallows will never appear, so better to take the one that's right in front of you and enjoy it while you can. Ergo, seemingly poor choices are often a response to poor circumstances, not evidence of stupidity or lack of moral fiber.
I also very strongly believe that people tell you what they value by their behavior, not their words. Her behavior suggests she doesn't actually want a house -- at least, not more than she wants the freedom to buy treats for herself every weekend. Really, if she wanted a house, there are many, many first-time buyer programs out there, many financial counselors out there who could help her figure out how to save up a downpayment and rework her budget, many loan officers who would be more than happy to find a way to finance something, and an infinite number of bloggers and columnists happy to tell her how to get there.
She probably "wants" a house in the same way most people "want" to retire at 40 -- it's a generic dream that you'd love to have, but you're not actually that enthralled by it enough to put in the work or make the sacrifices to get there. So her comments are more in the nature of a complaint about the unfairness of the system, or a pipe-dream wish, not a request for advice in how to actually achieve something. So when you tell her to give up the shopping, she already knows that; she's just decided that that's not a sacrifice she's willing to make -- that the house doesn't matter that much. And that is a decision she has every right to make for herself.*
In sum, it's not that she "may" have a reason that makes sense to her. It's that she
does have a reason that makes sense to her. Start there. Stop trying to say or do anything to her about how she manages her money until you can really truly accept that her thinking is very likely logical and valid given her circumstances and actual goals. Interpret her comments about owning a house as general kvetching, not an overriding goal that she just hasn't figured out how to achieve yet.
And if, at some point, she decides she wants to make a different decision, then you can be there to help her figure out how to do that. But until then, treat her like a mature adult who is fully capable of making the decision that is right for her -- even though it is not the one you'd make in her place.
*Of course, you also have every right to consider how much you're willing to let someone bitch about not being able to afford something when she's unwilling to take even the most minimal steps to help herself reach that goal. I personally have very little patience for that kind of thing -- if you want it, do something about it; if you don't, stop bitching about it.