Well, my boyfriend and I make about that much a year and I'm still broke. I have 65K in student loans and my graduate school costs $25,000 for my program. That's a huge expensive. Coupled with housing costs, gas, car payments, food and utilities, we're not as well-off as we seem.
As for asking you to pay, it doesn't matter if she makes 20K or 100K, that rude for her to ask and you should never pay for anyone. I might pay for a close friend's meal on occasion, if I knew they were having a rough time. But, even if I made a ton of money, it's not my responsibility to feed someone else......ever.
So, it's not right for her to ask. But, it's not right for you to question why she's broke. You're all in grad school, that's expensive.......you should know that. With that income, she probably doesn't qualify for any special loans or anything. She's probably paying her tuition out of pocket.
Next time she asks, just say, "sorry, I'm broke right now, I can only afford this." If you're in grad school, grow a backbone. You should have told her this the first time and she wouldn't have keep asking!
Wait to you get into the real world and people start asking you to buy $20 popcorn for their kid's school fundraiser........do yourself a favor, learn to say no and move on!