While prostrating in prayer, a man beside me raised my arm, thinking that I had placed my forearms on the ground; as such an act is forbidden. Was that man's action sound?

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 permissible for a praying person to alert another who prays beside him when it is necessary. This is considered offering advice. However, this should be done without any speech, many movements and turning away from the Qiblah (the direction of the prayer). 

Allaah Knows best.