Presumably, she already knows what her grades are, you did not promise to make any claims that were not true but you did promise to write the recommendation. The time to say no because you don't think she's a strong candidate or because you're too busy was when she asked and before you said yes. Similarly, you don't get a pass because you don't read email or attachments.
Let her know that, on reflection, you're concerned that your recommendation won't be as strong as either of you might like and offer her the opportunity to choose a difference reference. But be prepared to learn that you're as good a reference as she's got and that you will have to write the best recommendation that can be supported by the truth, no matter how much you'd like to get out of it. You promised, so keep your promise.
One way of doing this without stretching the truth or saying anything she might not expect is to focus on reporting facts rather than opinions, e.g., the actual grades and ranks in the classes she took with you and descriptions of the assignments she did for you.
Learn from this and think first next time before saying yes.