Should I decline to fill out a recommendation form after saying that I will do it?

I received an email from a student today. The email said that she was applying to a program, and that she needed her professors to fill out a recommendation form. She added that it shouldn't take long, because it should be easy to fill out. The deadline is exactly a week from today.

I am currently away from my institution, and will not return until her deadline, so this already puts me in a tight spot. She was also a weak student, maybe in the top half, but by no means a memorable one.

So I replied very quickly to her email (within 30 minutes of receiving her email), telling her that 1) if it's just the form, I can do it in a week; 2) if she needs a letter, I cannot write her a good letter within a week; and 3) overall, although if she insists I will do the job, she is advised to ask someone else for a letter.

She responded in a half-sentence, saying "it's just the form" So I asked her to send me her CV, transcript, and any relevant application materials. She has not responded yet.

I made a mistake here, though, because I was so busy that I did not open her attachment, thinking that if it's just a form, it should be easy to fill out. Now that I opened the form, I realized that I have seen this form before. I have already filled out this form for another student, and furthermore, I have written a detailed letter for her, because the form has an "additional comments" section that is very short (that I assumed had to be interpreted as a recommendation letter). I even asked about this letter here: Should I write that a student is a feminist in my recommendation letter?

So now I am facing some tough choices.

Should I still agree to just fill out the form for her? Given that I have already written a strong letter of support for another student on top of the recommendation form, this will be a death sentence to this other student's application since her grades on the form will be lower, and she will not have a letter to supplement it (however, the first student is a lot stronger in every dimension that I am qualified to assess them on).

I could email the student again, and tell her that because of the above reason, I cannot even fill out the form because it is too weak and that she will surely not get in.

Or, I could just do a half-assed job of writing a recommendation letter for this student as well, although the letter will have to be very generic and neutral, as I have none of her application materials, I do not remember her well, and I am also pressed for time since I am at a conference all week. Even physically sending the letter will be a pain since I do not get back until the day of the deadline.

So I am once again turning to you, hoping that I can get some quick answers (if I decline to write the letter, I had better let her know soon, so that she can find someone else). What should I do?