Only God knows his heart, and whether or not his bad experience was just an excuse to turn away from God. He may not even know himself; we seldom examine our real motives when we want to leave God out of the picture. This is why the Bible says, "Above all else, guard your heart, for it is the wellspring of life" (Proverbs 4:23).
At the same time, I'm sorry this church didn't measure up to his expectations -- or (apparently) to God's standards. No church is perfect -- which I hope you'll explain to your neighbor. But if you were the devil, wouldn't you try to get Christians to bicker and complain, or in some other way fail to act the way God wants them to? Of course you would -- because the devil's goal is to block God's work.
But when we set aside our own agendas and put Christ first -- both in our lives and our churches -- we won't give the devil this opening. Instead, our desire will be to live for Christ and be shaped by His Word. And when that happens, outsiders will see that Christ makes a difference, and will be drawn to Him. The Bible's instructions are clear: "Live in harmony with one another" (Romans 12:16).
Pray for your neighbor -- and pray that God will use you to point him to Christ. Urge him especially to forget the failures of others and look only at Jesus, for He alone is perfect, and He alone can save us.