I think you're overreacting just a bit. Read the first part of your first paragraph again. Now thank your lucky stars that you have such a great daughter.
Now re-read the second sentence of your second paragraph slowly. I don't think any of those things you're worried about actually happened. It doesn't sound like anybody pressured her or took advantage. She helped out a friend in a way she thought would get him through a situation that he was having trouble with. Reading X number of books, as such, doesn't sound like a core educational requirement anyway. It's a lot different from cheating on a math test.
I don't think I would punish this, especially if it's a first offence. You've got 7 more years of teenagerhood to get through; don't start building a reputation as The Adversary. What I WOULD do is help her increase her awareness of what school cheating can mean for everybody: the cheater (who gets credit for something he didn't actually do), the facilitator (who makes a lie possible), the teacher (who gets a false impression of what his students can accomplish, and maybe loses a chance to offer help to someone who needs it), and the whole school (which is trying to deliver grad certificates that actually mean something). She's only 13; last year she was 12. It stands to reason that she doesn't have a complete grasp of the big picture.
Your daughter was trying to help: great! Now she (and you) have the chance to think about how else she can help. For example, did her friend fall behind on his reading...
because reading's tough for him? She can help him get that improved, by encouraging him to use the school's resources.
because he's allergic to chick-lit about tormented vampires and Troubled Relationships and doesn't know about the Alex Ryder books or other books where interesting stuff actually happens? She can take him to the library.
because he's actually a fantastic reader and is 500 pages into a 700-page saga, but it only counts as "one book" and the assignment was to read 10? Well, in this case my sympathies are 100% with your daughter, but that's a conversation with the teacher that you should be part of, because clearly one size doesn't fit all.
Speaking of the teacher, you might want to consider a discussion in which you "don't reveal your sources" but you do mention that some of the online results may have been less than authentic. There may be a way to improve the process to make false entries harder to do, which would be a good plan if the same technique is going to be used for more critical assessment. You might also want to inquire what the purpose of the assignment was, and whether anything else happened around it (book reports, group discussions, class work about writing, etc. – or was it just "Read a bunch of books"?)
I think you need to know more about this situation, and how she understood it at the time, before you act. If you punish on the basis of what you understood instead of what she understood, that's another brick in the wall of My Dad Doesn't Get It, which is exactly what leads to more, not less, reliance on choices approved by peers instead of choices discussed with parents.