Not exactly a solution to the problem, but if he’s using bookmarks with reminders, and after the reminder is sent he don’t care more about the bookmark, he can delete it so as to not keep all bookmarks, that he probably will never see again.
He can even make it automatic by clicking in the gear icon when bookmarking and choose to delete once the bookmark is sent. This way, only the bookmarks with reminders not sent yet will stay, reducing a lot the number of bookmarks.