The fear of losing money will cause regret when you win a bet. In my opinion, it is just an illusion that crosses someone's mind because they are unable to bear the loss when they lose. In fact, if he was able to think objectively, winning and losing are part of gambling, maybe this small win would not have happened.
If you have that kind of emotion it's better to stay away, gambling always carried risk and if you can handle that kind of pressure then it's always a regret either you win or you lose you always have that kind of reaction that may triggered you to push foraward, and since fear is in you there's hesitation and regrets that cause of any decision making that you'll going to take. All in your own action as fear always carry that hesitation that cause mostly to lose your money.
With a reaction like this after a gambling outcome their is no need to keep on gambling if wining will lead to regret, then it means at first he shouldn't have gambled.
It is more better we make a good decision while making an attempt to gambling, so that whatever the outcome becomes lose or winning it will be easy to handle how one will feel about it.