The only problem with doing that (and I'd be very tempted!) is that you would then be outside of your terms with your internet provider as you would be 'charging' on top of their charges to you, which I'm pretty sure would be illegal.

You could point out the illegality of it so she's aware for the future and how much the Internet costs you during that time. Then put 'IF I were to charge you it would have cost you ..... to use MY service'. That way you're making it clear you have no intention of doing that. You never know what she might do (i.e. phone your provider and explain the situation).