The OP has provided conflicting information here, but they have stated that the person in question was both obese and tall. Perhaps the only two possible rows with some form of comfort for the person in question were the exit rows or Row 3.
Whenever I've traveled on QF with a comfort seat, seat selection isn't allowed, you can't get the front row and you can't get an exit row. I'm usually around row 6+ where the armrest can be moved up after takeoff (not that I need it)
Whilst in theory it would be nice, what if she didn't have the funds to purchase a comfort seat? This is a grey area with how it sits in relation to any anti-discrimination laws etc.
I think this is where it becomes the airlines responsibility.
That said, there's also the view that air travel
IS expensive and
ISN'T for everyone. It's not a right. If it's too expensive, so be it, don't travel by air.
I fear going any further into cost issues would take us off topic... but if a comfort seat on VA is too much (in addition to regular seat) then I'm willing to bet such a combo on JQ or TT would cost less and be the choice to make in the future.
Whilst I understand that, it would seem as if the Exit Row did provide some of the solution here. We don't know how full the flight was, so it is possible that the only two seats left in economy were there.
I am not arguing that it seems as if the situation wasn't handled as well as it could have been, but I guess we only have 1 side of the story (and it is likely we won't hear from the other 2 parties involved).
We rarely get the other side of the story on AFF, be it vital to form a conclusion or not.
However in this case, the OP wanted to know what the policy was and who to send a complaint to.
I think we've established there might not be a real policy, and the complaint should be informative but not emotive. I'd send it to the CEO personally, a PA will read it and get it to the right department.