Send a letter within a letter to the airline's personnel department. The inside envelope has your letter to your friend with a stamp, your return address, but the "to" address has your friend's name only. The street, city, state, zip is blank. The outside envelope is addressed to the airline's personnel department and has a note inside that says,
"I'm trying to locate so-and-so. I know you cannot release personal information to me, but if he works at your airline, could you please put his address on the enclosed stamped envelope and drop it in the mail. Then he can get in contact with me. Thank you."
I did this with a carrier several years ago and it worked. Three weeks later I got a call from the friend, a recommendation, an interview, and a job.