Back when I got inducted into the OA almost ten years ago I remember people telling us that the sash should not be washed because if it gets dirty it represents service. This led to some sashes that were in a really bad condition. Around that same time, the plastic sleeves were introduced but, at least in my lodge, it turned out to be a not so popular item.
A couple of years later wearing a clean sash became the norm, and we started to remind everyone that a scout is clean and a dirty and ugly sash didn't give a good image to the OA. About the sashes, i've never had any problems with the current sash when washing it with other pieces of clothing. Dry cleaning is a good option, but if he's just done Ordeal he'll probably want to do brotherhood in the next year so a new sash may be on the way. Then it's time to take care of the long-term sashes (I usually buy a new one every other year).