Does a Muslim who uttered a word of disbelief—Allah forbid—revert to Islam by praying?

The statement made by your friend, although unpleasant, is not considered disbelief (kufr) because he did not intend to disparage Allah Almighty, or curse Him, or mock Him. Rather, he intended to exaggerate his disbelief in you, even if the impossible were to happen. Your friend remains a Muslim, and it should be explained to him that such words must be avoided.