You have to decide if the friendship is worth more than the money. Often, it is, so you just accept the unpaid debt, and you think about all the other things this friend gives you.
—Rabbi Shmuley Boteach
I tend to let it go if the loan is small, say $20 dollars. But for $100 dollars or more, I say, "You know, it’s ironic but now I'm strapped for cash. If you loan me $100 dollars, then we'll be even."
What's that saying? Fool me once, shame on you. Fool me twice, shame on me. If you choose to continue giving the friend money, you kinda deserve not to be paid back. And she's not such a great friend, is she?