You should deal with this the way you should deal with any other situation in life: openly and honestly. Pretending it didn't happen or acting like you knew the truth all along wouldn't be honest, and would only compromise your integrity (and your witness for Christ).
Instead, admit openly that this person fooled you, and although your motives were honorable, perhaps you should have been more careful. Remember: He fooled you by lying to you, and he is to blame for your predicament far more than you are. Ask God to help you forgive him for the embarrassment he caused you and then ask God to help you put it behind you.
Let me add two further comments. First, pray for this person, and ask God to help you be the kind of friend he needs right now: one who urges him to repent of his sins and turn his life over to Christ. He needs God's forgiveness for the past, and he also needs God's help for the future. The Bible says, "In him (Christ) we have redemption through his blood, the forgiveness of sins" (Ephesians 1:7).
Second, don't let this experience keep you from being a friend to others in the future. All around you are people who need Christ, and He wants to use you to point them to Himself.