Your friend pushes you for making fun of him/her. What is the reaction force?

In anticipation of the push from your friend, you had probably set a foot back where it could push on the ground and be pushed back in a way that balances the push from your friend. You may also have given your body some velocity toward your friend, so that your friends push would cancel its momentum.

If your friend had dodged your attempt to push instead of pushing back by starting to fall, you might have either stumbled in the direction of your intended push, or had to shift your weight to your forward foot in order to let the ground push you back instead.