Self exclusion is the right term if you want to stop using your account. This feature usually available on your user profile settings or you can request this on the live support so that they will process this for yourself.
Request for a permanent self exclusion if you don’t have plan to come back anymore.
Delete feature on account is not available on any casino. Only self exclusion is the only option for this.
Not that true, there are some casino with these 2 features (self-exclusion and account closure).
Self exclusion is designed usually for those who have gambling problem such as addiction while account closure is usually for players who have no gambling issue but they want to close their gambling account for other reason.
However, I have to agree that if there is no plan to comeback then permanent self-exclusion is the best choice because as what I know is that account closure in some casinos still provide an option for the players to request account re-open.
Better to ask the support, that's would be the best thing to do for him and I'm sure the support will give him at least an option for his case based on his own preference.