I'm a fatty myself, but I can still fit fairly comfortably into a single seat. It's ridiculous to be so large as to take up two seats, and teen or not, fatty needs to pay for an extra seat if he can't fit.
I suppose if the lady was on standbye then they did what they should have in this situation, but if it's a full flight and there's nobody on standbye to bump then I should hope they handle it differently.