title_body: What to and what not to mix with alcohol
upvoted_answer: The main hazards of mixing alcohol with milk are that you may denature the milk proteins and get curds.  Unpleasant, but probably not likely to provoke an allergic reaction.
  
  Does your friend have any nut or pollen/tree allergies?  Depending on what the whiskey was casked in, there could be some residual allergens that were the actual culprit.