What do you say when someone calls you for a reference?

A former employee is now a friend of mine. She left the company and moved on to a few other jobs. She has the opportunity for another new position and has asked if she can use me as a professional reference. Flustered and caught off guard, I said yes.
The thing is, by the time she left here, she wasn’t a great employee. She was late all the time, took excessive time off and goofed off a ton during the work day. I get that she may have ‘checked out’ mentally while looking for her next job, but as her boss at the time, the change didn’t go unnoticed.

I’ve risen in ‘ranks’ here at the company where I remained. While the job she’s looking to get into is not in the same industry as the one I am currently employed, I have learned that this can be a very small world, both personally and professionally and am concerned.

I have no idea what her current work ethic is.. I can only assume it has improved from my personal experience with her, as she has done pretty well for herself, getting promotions, raises and better jobs along the way. If this new company calls me as her professional reference, what do I tell them?