Did he simply drop the lower unit to do the job? If so, pull it back off, put the control in forward, and the drive in forward, make sure everything is in place and lined up and reinstall. Simple!
PS - Make sure to turn the prop counterclockwise as you put the lower in forward gear - then keep pressure on the prop to keep it from going back out of gear as you install it.
You turn the shift shaft clockwise (looking down at it from the top - pictured below). Turn the prop counterclockwise until it locks in place. It is now in forward. Don't forget the washer - also pictured below - or you will be doing it again.
