Is it permissible to follow a person performing a fard (obligatory) prayer during his prayer?

This action is permissible, and one earns the reward of congregational prayer thereby. The one being led should stand to the right of the Imam if he is alone. If another worshiper joins them, then they should both stand behind him. This is based on the hadith of Jabir ibn Abdullah in Sahih Muslim, and the principle is that what is permissible in a supererogatory prayer is also permissible in an obligatory prayer.