My SO an I managed to get the companion pass on a promotion that they were offering. We expected to not qualify for a renewal into the next year, but at the end of November we gave a card to my mother in law who put some serious expenses on it. I just found out that because our statement for December comes in on the 2nd of the next month, none of our December expenses will count towards the companion pass. In other words, my mother in law's spending is not going to be seen by Southwest until January - at which point it will be too late.
It is possible to change the payment due date which would effect the statement cycle date... But it takes two cycles for the changes to be implemented. If we had known this earlier we would have changed the date in October to ensure that our December spending is seem by Southwest.
I just thought that this might be useful information for some future person looking for ways to improve their chances of qualifying for the buddy pass.