All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Offering Tasleem once after the two prostrations of forgetfulness is sufficient and what he did is correct. In Saheeh Al-Bukhari, Saheeh Muslim and others, it is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, performed two prostrations of forgetfulness – after he offered Tasleem in his prayer - then he offered Tasleem again without restriction to a certain number. Therefore, confining to Tasleem once is sufficient.
Allaah Knows best.