I don't see any reason why he can't move a follower to CC. The rules do only say that the player must not take part in any scoring. There's no indication that they must not be taking part in scoring at the moment that the completing tile is placed.
Moving a follower depends on two criteria: completing the feature, and scoring nothing. The two steps occur at different times in a turn, between which many things can happen. But in the end, so long as you fulfil the criteria, it's legal to move a follower to CC.
This circumstance could also occur with the dragon. You place a tile completing a city in which you have a big meeple and someone else has a single meeple. The tile has a dragon symbol on it, so the game is interrupted before scoring and your big meeple gets eaten. The other player scores while you score zero.
It could work the other way round—you complete a city in which you have a single meeple, and the other player a big meeple. You potentially score nothing at that point. But then the dragon moves and eats your opponent's meeple, and so you end up scoring.
And it could also happen with the princess as well; and with the tower.
What I'm getting at is that there are several situations in the game where a follower can be removed from play between tile placement and scoring. To say that the player must not be in the position to score anything at the moment they place the tile would leave it open for the possibility that they do end up scoring something; and to say that they must not be in a position to score on tile placement, and also actually end up scoring nothing, would be to add a third criteria to the rules.
I'd say that in the situation you describe, your opponent is just making the best of a bad situation (you stole all his points, after all). And there's nothing wrong with that
