I am going to bet on the number of years it takes for the couple to divorce.
Why do you want to bet on love like that?
I just think they got married a little too young.
While it's fun to bet around, it's especially rude to do it on a  happy relationship. You are invalidating all their feelings and hardships just for a night of fun and gambling. If they found out they'd be furious and not be your friend anymore.
I'm going to tell them.
It is not cool to make jokes about a marriage failing. I do not think telling them would be a great idea, and you should keep from making bets on something like this.