You're doing such a sweet thing for Bentley! Kaji was neutered around the same age and was an expert marker in his opinion.
Needless to say I didn't approve. Anyway, it's going to be a lot of work but if the family is committed, they can teach Bentley when and where it's ok to mark. To expect Bentley not to mark at all, in my opinion, is a bit far fetched.
What Bentley's family will need is patience, time, a belly band and incontinence pads. It's like potty training all over again. The incontinence pads are to help keep the belly band clean and a bit more sanitary as they are easy to change out. Bentley should not have free roam of the house if he cannot be supervised. When he does have free roam, the belly band goes on, and any sign of him lifting his leg, or squatting to mark must be corrected immediately. Also, all of Bentley's old favorite marking spots must be cleaned. If the house smells like pee, Bentley will think it's ok to mark inside.
One thing I did was encourage Kaji to mark outside when on walks. That way he knew it wasn't that I didn't want him to mark or pee (which of course is natural to them) it was just the location that needed to change. They can still be little dogs, and the owner just claims the house so to speak.