I sucked my thumb until I was 9. Everything my parents did to make me stop did not work. I became a closet thumb sucker. They had me wear mittens to sleep, bandaid on my thumb, shaming me. Shame was a big one for them. It only made me feel worse and made the habit worse.
Truthfully, I can't even tell you why I finally stopped but it was probably when my parents stopped harassing me about it.
I hope you were only kidding about asking his friend to tease because that seems downright mean.