Provided that your companion is the one on the "free" ticket then others have successfully flown by themselves for parts or all of the journey under the Amex companion ticket program. See:
http://www.flyertalk.com/forum/briti...r+return+alone
In fact ba.com helper (Dilly who has moved on) confirmed that an Amex companion ticket holder can fly without their companion:
Hi,
I've checked and would like to confirm that you can travel without the companion on the free ticket.
However, they cannot travel without you.
Kind regards,
Dilly
http://www.flyertalk.com/forum/11446410-post17.html
In fact the person helped by Dilly traveled the entire itinerary by themselves.
See the entire thread here:
http://www.flyertalk.com/forum/briti...g-tonight.html
See also:
http://www.flyertalk.com/forum/briti...1-booking.html