Is the prayer of one who intended a Sunnah prayer valid, and is he sinful, if another person followed him, thinking it was a Fard (obligatory) prayer, and that person completed the prayer without being informed of the leader's intention?

The most sound and authoritative juristic opinion (fatwa) is that it is permissible for someone performing a fard (obligatory) prayer to be led by someone performing a nafila (supererogatory) prayer, or vice-versa. Consequently, the prayer of the person who prayed the fard behind you while you were praying a nafila is considered valid, and you bear no sin for it.