Avoiding discussing a recommendation

A simple declination to discuss should suffice.

Hi (Person),

I will not be able to discuss (Mr/Mrs student). Sorry for the inconvenience.

Sincerely, (Professor)

Don't make up a reason. It only gives someone an opportunity to continue asking if they have a way to mitigate said reason. Also, if it is indeed a nefarious as others have suggested, there is nothing for them to take and run with.

This smells like a trap. I tend to be more paranoid than most, but this has often paid off. Could it be that this potential employer is really a friend of this student and they're trying to get damning evidence recorded to aid in a planned lawsuit? These manipulative types are adept at spotting people who are easy to manipulate, which would explain why the student originally insisted on your colleague writing a letter, even after he'd been put off. Now he's back with another scheme. That your colleague feels uneasy means that he's aware, on some level, that the student is predatory. My opinion is that he should run away from the situation. He might state that he has a policy against doing any sort of recommendation for anyone he hasn't dealt with in 5 years, since his knowledge would be so dated.

If this is in the United States, in the absence of a FERPA release signed by the student, your colleague cannot discuss anything that's protected by FERPA and should not discuss anything that is arguably protected or might be. It's not just a good idea, it's the law! (But this isn't legal advice.)

FERPA contains an exception for recommendations to schools to which the student has applied. That exception does not apply to potential employers.

The right answer is, "I'm unable to discuss former students."

Edited to add: I was being precise when I wrote "former students;" don't say anything at all about this student. And, as noslenkwah has already written, don't give a reason.