I found a couple questions such as this one that ask what a job-seeker should do when they use a reference without permission, but I find myself in the opposite situation. I was just contacted for information about someone because they used me as a reference unbeknownst to me. I know of the person, but not enough that I could endorse them when asked about their character. There's not a way for me to get in contact with this person anymore either.
It was definitely a mistake on the applicant's part for putting me down as a reference without 1) me knowing them well and 2) contacting me, but I would hate to sell them out if they were otherwise suited for the job, provided real references aside from me, and then jotted down me because they were put on the spot in an interview.
Should I just ignore the request for information and let their other (potentially real) references do the work, or should I be up front and tell the company I don't know the person well enough to provide any information? If the former, would one of the references not responding affect the hiring process at all?