First, I would let go of today's loan. She had already decided to give her friend the money, and probably told him that, so in this case she wouldn't have felt able to not do so, even if all your (newly presented) words make sense to her.
Soon, I would ask her for time to talk about moving toward shared finances in future. Your words above show that you're in for the long haul, which is often good news to people in a relationship.
Then, work hard to ask, explore, and understand her culture/ideas around loaning/giving to friends and anything else financial. When you understand, reflect back everything you've understood, then ask her if you missed anything.
After you've had some time (days) to ponder that, say what's true for you, and ask how you two can move toward building a shared financial life, if that's what she wants too.
And, don't call her friends idiots, selfish, or anything else disparaging. It's not only disrespectful of her friends, and of your beloved's heart choices, it puts her on a psychological defense and kills any chance at awesome conversation. Her friend's financial moves are not the issue here. Your beloved's family culture dictates that she give when a friend expresses need. That needs to be navigated within your relationship; the wisdom of her friends does not.