First - if you are interviewing, keep yourself focused on the fact that you want to make a great impression. Trying to work in a bid for a collegue could potentially distract from the fact that you would be great for the role you are interviewing for. Let that be the primary purpose.
Next - even if you work there, most work places will require that your colleague actually submit a resume or application. If your colleague isn't ready to do that, or hasn't done it - stay out of it, there's no point distracting folks with a potential that is unlikely to be realized.
I'll say that I believe I have, or would, mention a potentially good recruit, but only in certain cases:
1 - I'm talking to the recruiter and it's going well - the recruiter is most interested in helping get good recruits, so he may be more networked than anyone you interview with. If you see the recruiter at the of the day - mention it there. If not - mention it when you get a chance, if there is time.
2 - I'm talking to a manager who has expressly mentioned the need. I don't try to work it in. Admittedly, because I myself interview for management jobs, it's not unusual for a good conversation with a boss or boss-peer to involve the discussion of the challenge of finding great people. I can remember at least one time where a potential colleague said "yeah, we have so much trouble hiring XYZ". I was able to say "do you need A or B or C, too?" - we clarified the need and I actually knew someone. I gave the info, and then never followed up. It's not my business if I'm not actually working there. This was a good case, in that I was also showing that I have a great network, and I'm interested in helping -- but it would have seemed different if I had done the same thing without the intro from the interviewer.
Avoid if it you feel you have to reach for it awkwardly. The awkwardness will show, and won't leave a great impression.
Do have the colleague's contact info - business card or resume - that you can give if you actually get some interest. If you don't fit it in, then at least you were prepared, no harm done.