I htink the SOuthwest policy is a fair one. WHile I feel for people who cannot help/control their size for medical reasons, it stands to logic that if I were not 'comfortable' on a plane, I could not say, put my feet over onto the next pax's lap to give myself more room (extreme but I'm sure you'll see my point) as it would not be fair as they have paid for, and are therefore entitled to, that space.
I think the same applies to my seat. I have paid for it, so I do not want some 'person of size' spilling over onto me/my seat while they sleep, thus depriving me of MY room and comfort. I do appreciate the ones who try not to take up more room than they can, but perhaps this would be the way to go for the airlines. Maybe for those who have medical conditions and are genuinely 'large' for that reason could have a card entitling them to some sort of rebate.. maybe then would encourage those who are big through poor lifestyle to either re-think travelling by air or makes some positive changes to their lifestyle...
I do agree though, that this matter should be dealt with tactfully by airline staff... measuring him at check in is probably a bit too much. Would have been nicer perhaps to have taken him to a private area and explain the situation... or have a 'test seat' for people to check whether they require to purchase another (reduced rate) seat??