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.
It is recommended to tie the jaws of the deceased after the soul departs so that his mouth does not remain open. This recommended principle was legislated as such due to a non-obligatory command or it is that which is deemed rewarded when done but not punished when abandoned.
Therefore, what your friend did is not considered a sin since there is no sin for violating a recommendation. Furthermore, he would be excused due to his ignorance. We only advise your friend to learn the rulings of his religion.
Allaah Knows best.