Answer
If you love your wife, then you should do what pleases her.
Now that she is no longer physically present, the best way to honor her memory is to continue the devotional activities that were dear to her. If she used to come to the temple, then you should come to the temple. If she engaged in seva, then you should engage in seva. If she used to donate, then you should donate.
In this way, you will feel that you are concretely pleasing her.
Rather than immediately introducing philosophy or even Krishna consciousness, first connect with the person's existing attachment. Help him reconnect with his wife through the devotional activities she cherished.
If I were in that situation, I would try to connect him more and more to his wife's Krishna conscious qualities. Through that connection, he can gradually reconnect with Krishna consciousness itself.
