Your friend do sound very immature to be honest. Why mock when competing with each other? Was it like a "strategy" to distract your friend so he loses the bet? If that is the case, then tell him it's not a fair win.
I think it's just some sort of making fun, like shouting the loser is loser, it gives pressure, but it shouldn't be use as an excuse if he lost.
The lesson of this story should be that it is not wise to gamble with close friends, some would say that it's also not wise to lend money to friends... that is how friends are lost. I know from experience that this can be very true, of course, there are exceptions and responsible people and friends, but some are difficult and they show their real face in situations where money is involved, especially with some serious amounts.
Sometimes I keep thinking what's the benefit of having friends? I think when a friend is genuine ask a loan, he's really struggle and has no way to figure it out except seeking help from his close relative.
If we're limiting those "personal help" to our friends, what's the difference between friends and strangers?

This happens especially on the cards, Chess and the other skill-based gambling.
Chess isn't a gambling game. Do you play Chess for money(or bet on Chess games)?

Same as cards, even you're playing poker, you don't have to stake anything to play. Actually chess is a games that can be used to gamble, many casinos listed chess in their sportsbook.
