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Are you advocating this poor man use a "stealth" approach to determine whether the co-worker he is in love with has any feelings for him before he acknowledges any of his own?
What good is it going to do him, his wife, or his co-worker for him to acknowledge such feelings to her?
You mentioned that it will be best for him to find out her feelings before "acting" on any of his own.
How are you advising that he act? If he declares his love for her, and she acknowledges some love for him, and the two are married to other people, this is just not going to end well.
Perhaps this man should explore WHY he is having such strong feelings toward another woman at this time. What is life like at home? Is he feeling disconnected there? Turning so much energy toward a coworker is not going to do his marriage any good.