My friend has a milk allergy so can she just take lactaid and eat pizza?

Possibly. Milk intolerance is most often not an allergy but a deficiency of the enzyme (lactase) that breaks down lactose (a complex milk sugar). If it is not broken down it is poorly absorbed and bacteria act on it to form gas & acids and abdominal pain. Several things can cause low lactase. I recommend trying to eliminate milk products if that stops it, try lactaid and slowly add milk, it may work.