You have a few options and I'd say something like the following:
I'm sorry but I don't feel comfortable recommending you because... [pick one of:]
- we never worked closely together in workplace setting so I can't credibly vouch for your work
- I've only been with my current employer for X months
- I don't know enough about the position/team you're applying for
- I feel uncomfortable injecting myself into the hiring process as I don't want personal bias to affect the selection process
but I wish you all the best in your search.
If the nature of your relationship allows it, you would be doing your friend a real kindness if you were honest with her though. Some people go years without the kind of feedback they'd need to establish a professional attitude and workplace ethic. Some people can't be helped but many that were poor students or unreliable team players in college become great employees with a bit of guidance.
Some of these were adapted from this article by Alison Green which I highly recommend reading. You should also keep her advice here in mind:
First, be sure that you really think she wouldn’t be good at the job — and that it’s not just that you don’t want to work with her. Because if it’s the latter, I can’t condone standing in the way of someone finding employment based just on a personal dislike.
