It is a little unusual for an adult to have a previously unknown food allergy with this severe a reaction in someone as old as Hitch, but not impossible.
Treatment was appropriately given with benadrylin the movie, but in reality Hitch additionally should have gone to the ER where epinephrine and steroids also would have been administered followed by several hours of observation.
After recovery, he should ALWAYS carry an epipen with him. He should also probably see an allergist to determine what it was he reacted to so that he can avoid it in the future.
On another note, in the movie, Hitch literally chugs down an entire bottle of liquid benadryl and acts drunk from it. That usually wouldn't have happened... rather, he would have gotten extremely drowsy and fallen asleep fairly quickly. However, in some individuals, it might occur.