Each passenger has their own e-ticket. PNR's are irrelevant. If a passenger is a no show, his ticket is cancelled and the ticket retains whatever value it retains under its fare rules.

The only exception to this are special features where a given passenger must be present for the other to travel. Not your situation.

I would send an email to BA with the particulars and then print that out and take it with you to the BA counter. It at least shows that you attempted the cancellation. No idea on logistics as we don't know your airport.