Benadryl helps for:

Itchy Watery Eyes
Runny Nose
Itchy Throat
Nasal/Sinus Congestion
Nasal/Sinus Pressure
Sinus Pain
Sinus Headache
Sore Throat

It is besically a histamine blocker which unfortunately also is a sedative. It will only work to certain extent. First of all you have to make sure that airway is open so he can take a tablet. If he is in severe allergic reaction oral benadryl is useless. In medical field benadryl is used as an adjunct to Epinephrine treatment and form that we use is liquid for IV or IM injection.

Now if you are suffering a minor discomfort in form of sign and symptoms described on top of this reply that benadryl will work. But if you just swell up like a baloon and cant breathe unfrotunatelly more serious theraphy is advised including agresive airway managment and epi treatment.

Also benadryl has an atropine like action which may trigger asthma attack.It causes drowsiness and that may be something you don't want if you are getting out of place where full attention is needed. SInce it also opens you up but speeds up your heart you have to be carefull not to bottom somebody out which means drop their blood pressure below minimum and make sure they are warm since increase in circulation will cool your body faster.

Now above is probably more than you wanted to know. If you have space in your bag throw it in. Most likely person will know what they are allergic to and be prepared. I've seen benadryl making a difference when it changed a horrible trip to a decent one by pluging somebodys nose and clearing up their eyes.

Also make sure that somebody who you are going to give it to is not alregic to it or other anti histamines.