Sadly, I don't think there is any hope here. The only way to guarantee that your travel companion will travel with you would be to buy (revenue or Avios) a CW ticket.
Any expectations that BA will show some sympathy and therefore make an exception is, imho, unrealistic. Unless the boundaries in these circumstances are clear, not only would it be unfair towards other passengers, but it would just be non-sense from a commercial point of view.
If you don't want to pay for CW, the only hope is that your travel companion will be lucky and receive an operational upgrade. These are rare though, so it is very unlikely.
Even if you check-in online for the ex (which I don't think you should do unless you have her permission), if for example you travel out of terminals with conformance, the ex will still be offloaded before the flight and his seat will be potentially given to someone else.
Last edited by ringingup; May 25, 2018 at 6:01 am