How to deal with a procrastinating (student) collaborator?
There are a couple of options that you could consider:


You may have to put your foot down (in a nice way), and gently remind her that it was her idea to collaborate and that such collaboration is difficult unless you both adhere to the deadlines set.  If it still persists, tell her that the procrastination, particularly with the tasks are making the collaboration untenable.
If this is not a critical aspect of your work, take a more relaxed attitude yourself to it - in terms of punctuality and task-completion.  The rationale for this is that if she does not place the collaboration as a high priority, then why should you?  This second one you would need to be careful about, the approach would be to put your other work before it.