References should honor privacy, but be forthright if they screw up

Questions revolving around writing references seem toarise frequently. More than once I've received questions about whether it's OK for recommenders to ask you to write your own recommendation to which they will simply affix their name (it's not).
Now, the question arises of what the right thing to do is when someone writing a reference for a former colleague is not as careful with keeping that colleague's decision to apply for a job elsewhere confidential.
Here's what happened.
A reader, let's call him Reed, received an email from a former colleague, Colleen, telling him that she was planning to apply for a new job and wondering if he would be willing to write her a letter of recommendation. He emailed back indicating he'd be glad to write the letter. Colleen sent Reed some details and he subsequently set aside some time to write the letter.
A few days later, after Reed had written the letter, a copy of it was sitting on his office desk, waiting for him to sign it, stick it in an envelope, and send it off. (Reed is very efficient at getting recommendation letters out in a timely fashion.)
That day, however, Bart, another former colleague who had worked with Reed and still worked with Colleen arrived at Reed's office door. Reed had forgotten that he and Bart had agreed to meet for coffee and to catch up. But there Bart was at Reed's door ready to go -- and there sat the letter for Colleen in open view on Reed's desk.
Bart plopped down on the seat next to Reed's desk and they began chatting before taking off for coffee. Well into the conversation, Reed realized that Colleen's letter was sitting in open view. He reached for it, turned it over, and made a comment to Bart to the effect of, "Sorry, I shouldn't have left that out."
"That's OK," Bart joked. "I learned to read upside down a long time ago." At least, Reed hoped that Bart was joking.
Now, Reed was concerned. He promised to keep Colleen's job application confidential. He didn't think Bart had seen anything or that he would say anything even if he did, but he wasn't sure whether he owed it to Colleen to give her a heads up.
Reed should have been more careful with Colleen's letter once he printed it out. Granted, she didn't work at Reed's new place and no one there knew Colleen, but given that it was a confidential letter, he should have taken a bit more care.
But Reed slipped up, and he wanted to do right by Colleen since he had written her a strong letter.
If Bart did see anything, he should have told Reed that he did. And even if he didn't tell Reed, he should not disclose to anyone what he saw. Friends should not betray friends.
Reed isn't obligated to do anything. He is, after all, doing Colleen a favor by writing her a letter. But Reed chose to do the right thing by emailing Colleen and letting her know what happened. As far as either of them know, Bart either never saw a thing or, if he did, kept it to himself. 
Jeffrey L. Seglin, author of  The Right Thing: Conscience, Profit and Personal Responsibility in Today's Business and The Good, the Bad, and Your Business: Choosing Right When Ethical Dilemmas Pull You Apart, is a lecturer in public policy and director of the communications program at  Harvard's Kennedy School . 
Follow him on Twitter: @jseglin 
Do you have ethical questions that you need answered? Send them to rightthing@comcast.net. 
(c) 2014 JEFFREY L. SEGLIN. Distributed by TRIBUNE CONTENT AGENCY, LLC.