In these situations one cannot just make an attempt and cease doing it if it doesn't work.
If you have no "handle" where to keep yourself your attempt to lean over the embankment to lift the other person can easily make you fall inside the trap. The man is grasping your hand and pulling you toward him with all his weight (as he's lifting himself) and you will not manage to lean backward. Gravity will work against you "big time".
A different case would be if, say, the man already had a knee above the embankment and you only have to help him for the last push.
For me it would be basically impossible to lift a man who is inside a hole up to his chest just by the sheer force of my arms/legs.