I’m next to an enemy and he moves 1 square out of my range. I hit him with Opportunity Attack and have Sentinel. Did he move 1 square or not?

We use grid on our games and have a question about aoo and sentinel.
Say im next to an enemy and he moves 1 square out of my range. I hit him with aoo and have sentinel. Did he move 1 square or not? His speed is now zero, but was him able to this one square?
thxPlayer's Handbook pg. 195 covers opportunity attacks:

"The attack occurs right before the creature leaves your reach."

If they are right next to you and try to move out of your reach, you hit them before they move, and your Sentinel feat reduces them to 0 speed. They stay put.

— Dan Dillon (@Dan_Dillon_1) April 17, 2020

I thank you very much for taking the time to answer me.
And i’m sorry i bored you and your friend. Not boring at all, I'm happy I could help!

— Dan Dillon (@Dan_Dillon_1) April 20, 2020