Antipsychotics work on a lot of drugs, although you should do some research before giving them to people because they can be disasterous mixed with some drugs (DXM + Seroquel is one example). I've only ever used them once, when I was a fairly novice tripper and on a rather high dose of DOM, my friend gave me Geodon to calm down, and it worked wonders.
I wouldn't recomend taking benadryl while having a bad trip, it could make you pass out, but it wouldn't end the trip and would in all likelyness make it stronger. It might work for some, but giving it to someone who was freaking out would be playing with fire in my opinion.
There are ways to chill out trips that don't require prescriptions, the person in question could take Holy Basil, Kava Kava (booze would work just as well if not better, and this one would probably dull if not end the trip, similar to benzos)or L-Theanine to try and relax them. Opiates wouldn't end a trip, but they could turn a bad trip into a good one.