Pure speculation on my part, but I would imagine that if your companion gets caught in traffic en route to the airport, misses the flight and elects to find a different way home, then it's no different to any other no show on the same PNR - i.e. no big deal.

If, however, you phoned up BA in advance to cancel your companion's ticket, then you may run into problems. But, logically speaking, why would you? You've still paid in miles for your ticket, it's the 'free' one you're cancelling.