Oh, I think you're fending for yourself. You shouldn't have to be her mother, she's your roomate, for goodness sake. Not your kid. If she wants to waste her money like that, she deserves to be constantly broke. DO NOT under any circumstances, LEND HER ANY MONEY! I'm sure you already know that, though.
Let her go to the public library to get on the 'net. And yes, you should get cable and DSL....but also be prepared to get a lock put on your door. She won't believe it when she finds out, she'll just think you "forgot to tell her" and she'll stroll in there wanting to use it, especially when you aren't home. No free rides, and the sooner she learns this the best.
You don't have to be mean about it, when she asks, just point out that you are as financially strapped as she is, and you can't afford to carry her, as well as hersefl. When she starts giving you money towards the internet on a REGULAR basis, she is welcome to use it.