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.
I second what @panjulo07 say, account closure is one of the options in casinos if one wants their account shut down. Although account closure option is not seen on the page, all the member needs is to send a message to the support staff and request to close the account and the staff will process the request. I have done this myself when I accidentally created another account on one gambling platform since the first account I created is through normal registration and the other one was accidentally created through gmail linking.
While the self-exclusion is a restriction of some of the account activity within a certain period of time, the owner of the account can still access the account but I believe is limited to log-in or fund withdrawal. Bet and deposit is usually disabled during the effect of self-exclusion and can have a full access of the account again once the time of self-exclusion expires.