It’s worth it to know that there are genetic conditions that may affect the way an absol’s horn grows, so there’s a possibility that this trainer is right in that regard. Alternatively, the absol could be a rescue whose previous owner was abusive, and its current owner simply doesn’t want to talk about it. If you suspect abuse, it’s important to look for other signs, including malnourishment, fresh injuries, or strange (anxious, aggressive, etc.) behavior. Making an incorrect call on such a matter could put an owner and a pokémon through quite a lot, some of which can be highly traumatic for a rescue.
Still, it’s good to hear that you’re concerned for a pokémon’s well being! Please continue to care as deeply for pokémon as you do.