5% is a decent safety margin in order to not lose control over financial expenses with gambling. Some gamblers still go beyond and adopt a 10% from total income strategy, but they must keep in mind to be actually sacrificing money they could be spending with other leisure activities, potentially side by side with their families.
It's a personal decision each one has to make, and nobody else has anything to do with this, but it never hurts to give benefical and worthy advices to people who we encounter in life, so we can try helping them somehow, especially in this case where we can advise them to risk less money from their total income with gambling.
10% is a lot for gambling, unless it is a one-time contribution, which you will play with without monthly replenishment. And by the way, everyone's salaries are different, so you should not judge only by the percentage ratio, but in any case, you need to approach the gambling budget carefully so that it does not affect other areas. In addition, I am a supporter of the idea that we should put aside part of our income from each salary for further investments, and gambling should not affect this either. So limiting risks in gambling begins with determining the budget, if this is done correctly and without damage to your finances, then this will not entail anything bad even in the event of losing this money.