How can I refuse to "help" my friend without sounding like an asshole (looooong details inside)

A bit background: I have a friend who is really fun to be around. But one of her flaws is her compulsive spending habit. Each month she would spend almost all of her money on things she can’t really explain why they’re necessary and would sometimes complain of being “poor”, although she works much more than me and I’m sure the money she earns is enormous. I know she has to pay the bills and stuff, but the speed of how her money drains out each month just bugs me.

Now the story: some time ago she talked to me and another friend about this mobile app that helps you pay for things from far away. I’d heard about that app long before but I never intended to download because I didn’t need it at all. And I knew the only reason why she brought that app up was because once I downloaded the app, she would be given $43 in her account, an award for inviting a friend to the app. I just said “let me think about it” and we quickly forgot about the app.

Cut to this morning. I received a message from her. She asked me to download the app again because she was in dire need for $43 to buy some books. Now that message put me into a weird situation. I want to help my friend, but I don’t want to download that app just to give her emergency money. Beside, $43 is a small sum of money in the long run and she could quickly spend it impulsively, and the last thing I want to do is to enable her spending habit. But I don’t want to sound like an asshole who turns her back from a friend in need.

I still haven’t replied to her message. What should I say to her?