I would have to agree with the previous poster and say that your former summer associate probably filled out a work history and listed your name and number with no expectation that her recruiter was actually going to contact you. Also, if you are not thrilled with her work she probably has an idea and wouldn't put you as a reference. :-)
A few years ago, I was faced with a similar situation for a summer intern. She wasn't horrible but she was probably average to slightly below, BUT she did come through for me when I really needed her to. She asked me to write her a recommendation for a top school. I absolutely REFUSE to lie but I just wrote down all of the positive things that she did do and focused on those aspects. (She was accepted.) I figure that this is her chance to change....and no one appointed me judge and jury. :-)
Your never know what is going on in the associate's life...personal problems, maybe the job wasn't a fit etc. Now, if your summer associate was really bad e.g. lost you a client, plagerized, stormed out of a meeting with partners/c level officers...i don't know came to work drunk...this is a different story. If she was just average though, I would give her the benefit of the doubt. The job market is tough and, ultimately, she could have changed and matured in two years. The ultimate hiring decision is up to her future employer you are just allowing her to get her foot through the door.