I am thinking evil thoughts here but what if I make my friend fail english on purpose? She ask me to help her so if she fails.. I will laugh at her and she gets no aid just so she can feel what I feel.. That is wrong I know!
I personally think you should explain your feelings to her nicely. Help her anyways because that's what friends are for. You should do things for her knowing that you may not get good things in return. Your support for her should be unconditional.
And about the fact that you know it's wrong, don't worry about it. We all have our moments where we think things we know isn't right. It happens to me a lot. So try to resist that urge as best as possible. It's okay if you fail, we all have our mistakes! :)
With friends like you, who needs enemies.
It's a shame I can't warn your friend about what a jerk you're being, but karma has a wonderful way of biting people in the ass.