I asked the state coordinator of officials in Nebraska (NSAA) regarding the situation in the OP and this was his response:

Probably should have skipped the technical foul. The player that dunked the ball was not part of the game being contested. He and his teammates shouldn't have been on the floor.

According to the NSAA basketball manual, teams are not permitted to shoot/warm-up at the intermission of the preceding game. In this case the athletic administrator should have been informed and the violation should have been reported to the NSAA.

Moral of story: Check with your state on how to handle this, they may already have a policy in place.