Kind of like a "Living Will" - the clause only kicks in if someone is actually incapable of speaking for themselves. The "minder" role is a safety-net of last resort - for if I screw up and let myself go too far. Under usual circumstances the "minder" role never comes into play and he would just be the "designated driver".
Again... maybe he knows something you don't about her in her subspace. She may not be capable
Maybe this could help you see a taste of that side of it.