In my opinion, a trip should only be aborted if the person has gotten into a dangerous situation. For example, if they've gone totally psychotic and are wielding weapons, or trying to hurt themselves, or if they've totally lost all contact with themselves and the world, then aborting the trip has become a matter of physical safety.
However, if a person is just having a horrifying trip, but they're not in physical danger, then in my experience it is definitely better for that person to ride it out. The thing about "bad trips" is that the "badness" is there for a reason. As long as the tripper is conscious of what is going on, that horror is going to teach them something about themselves. Every horror trip I've been on has ended well. Once the peak wears off and you begin to return to normality, the weight of the world is lifted, and you end the trip on a positive note, happy to return to reality, mind reeling with what has just happened. Then a period of sober reflection occurs, and the person emerges better and more understanding of how to fix whatever it is in them that caused the horrifying trip.
If, on the other hand, you're having a horrifying trip and load up on benzos or alcohol or whatever to abort it, then you're left with the negative, horrifying, semi-psychotic impression of the trip gone bad, but there's no natural transition into that introspective mindstate that you get upon re-emergence into reality. So all you're left with is this lingering horror that was never resolved. And it's likely that it will come back to haunt you, at least subconsciously.
So in short, aborting a trip should be done for physical safety reasons only, IMO. A trip has a way of working itself out, if you let it. And like all difficult experiences, if you face it, it will make you stronger and wiser.