Originally Posted by Deltus

If you do decide not to cancel it, you can probably maximise the chances of it staying empty by checking in, downloading the boarding pass, entering security on their boarding pass, returning landside using the transfer shuttle, and re-entering security on your own boarding pass. All the systems will assume your companion is flying right up until the flight closes and nobody has materialised at the gate, by which time it will probably be too late to fill the seat.

But a refund of the taxes might be a more attractive option.