Can a dog be stronger than me?

At the first, no you would not be able to hold him back. When the dog first takes off every muscle of his is used for that burst of speed, and you would not know exactly when it would happen. You would probably be able to control him after ( at a great cost of strength mind you to yourself) You have to remember though, you may weigh more than him, but his body is made up of almost 120 lbs of muscle. Strength does not always relate to size either. That and animals use their muscle in different ways, they are placed different in their bodies, more compact, greater. So all in all I would side with your friend.