The rules wrote:
Players at their limit may destroy friendly cards to make
room for a new card. Players may not destroy a card from
their city or army unless they are at their limit.
So he could destroy a building he already had in order to make room for a cheap building just to deny you resources, but then he's stuck with that building until next time he does this and he's potentially wasting one or more building slots just to spite you.
I've seen players destroy a farm just to replace it with another farm though just to deny the resources to downstream players. Seems totally legit.