I am not sure you can do much about your partner's seat selection. I also feel unusual that you could select your seat online for an award booking too.

CX has been "selling" the extra legroom seat (except for MPC elites), but I don't know if that is the case for KA. If you are willing to pay extra (but USD100 is a lot for a 2-3 hr flight), perhaps you can ask if you can get an extra legroom seat.