Dog allergy symptoms are red, itchy watery eyes, nasal congestion, runny nose, sneezing, post nasal drip, difficult breathing, coughing and wheezing, itching and hoarseness in the throat and reddened areas on the skin.
Dog's dander, urine and saliva are allergens that cause allergic reactions, and one dog and another of the same breed can give off very different levels of allergen.
Every new contact or exposure to dog's allergens will lead to more intensive and noticeable allergy in allergy predisposing people (so, if your friend doesn't show symptoms for now, it doesn't mean he 'll be symptom free in close future).

There are some tips how to lower your dog's allergic potent and how to make your environment free of allergens.