Well it probably would not have been legal (although it should be legal,
and it will be legal soon) but what you could have done, would be for you
both to exit at St James Street (there's no barrier is there?) and then
after you do your Sainsburys shopping together, walk to your house and you
go in Walthamstow with your season ticket, and exit with her oyster (oyster
pre-pay are transferable after all). Journey resolved. OK it took 20
minutes longer than it should have done, but I bet the machines wouldn't
think that's particularly odd.

Probably unethical, but you're only doing what LUL would like you to be
able to do but don't have the technological ability to do it yet.

(queue the sound of angry UTL people saying I should be strung up and shot
for suggesting fraud).

Disclaimer: it's not what I would do - I have a season ticket. But it makes
logical sense, doesn't it?