It depends on how old it is. Up until about 5 years ago, many Arai's, (as well as many other helmets), only had removable cheek pads. The top padding was glued to the styrofoam. Many of the top helmets were like this because the plastic snaps needed to hold it in place were seen as a potential pressure point. It wasn't until a lot of testing was done that the liners were made removable.All the liners in the better helmets are removable. Take that out, put that in the dishwasher or just buy a new one.
My old Arai SC Z/m lasted me 7 good years. Even if I could replace the liner, the leather strap is so worn that is would be a waste of money.
I think what he does should be dictated by what the condition of the rest of the helmet is.