You could let the guy know that you don't feel comfortable giving the name of the person but someone from his office applied with you and that if he values certain employees then he should be open with them and probably compensate them to an appropriate level if he can't afford to lose them or structure his business so he can lose them.

You could also call the applicant and just be frank with her. Tell her that you don't really have any duty not to disclose this but you think it would be better if she just is open with her boss about whatever it is in her situation that is making her look elsewhere. You could even give her a hint that her current boss really values her.