I've definitely forgotten my wallet and/or had no way to pay on me before. It's embarrassing every time, even when I can just easily run 5 minutes back to my desk to get some money. That said, I certainly don't make a habit of it and I've made it a point to pay anyone who covers me back every time and still a pretty good number of people will say don't worry about it and not accept the cash.
So I can see where someone without scruples will hear "don't worry about paying me back [this one time]" and think it extends to all future loans. Or not even wait for someone to say that and just assume, "oh he/she won't miss that $5, or $10, or any amount anyway, so no need to pay it back."
Personally, I don't get either the LW's or the AW's mindset. Clearly LW should just say no. Just, no. Ideally she would even say something like "Well AW, you still owe me for the past X lunches, so I'm not able to loan you any more money." But if she could do that, she already would have and the problem would have gone away.
I kind of wonder that none of the friends have said anything about it (privately to LW), but they may not realize that AW doesn't pay LW back later. If they do know AW is a mooch and didn't warn LW or just watch it happen every time and say nothing, the LW should really find new friends.