I have a LHR-DXB-LHR flight paid and booked. I now need to buy MsFF101 a one way DXB-LHR flight. She'll just join for the return leg and has no status.

I want to buy her a cash ticket and upgrade using avios. There is availability. Can I buy her a ticket and upgrade using my avios? I'm actually in a household account that she isn't part of (long story) so does that complicate things?

Secondly, can I link her booking to mine so we can get book seats together for free?