"Relax, it was just a joke!" You try to make him calm down with your lie.
Your prey responds slowly. "R-really? I thought you were a-actually going to do it..." The struggling and squirming stopped, showing he had calmed down and that he believes you. However, you still will digest the other Riolu. You smile at the thought.
You slowly drift off to sleep with your large meal not struggling at all.