3 month old allergic reaction to ice cream

We were @ a wedding receptiion and my sister was holding my 3 month old daughter. Next thing I know she is feeding her ice cream...mainly letting her lick the spoon and rubbing it across her mouth. with in just a few minutes my daughter had red hives all over her face and neck!!! very scary. We were instructed to her a 1/2 tsp. benadryl and monitor her closely for an hour. then repeat dose benadryl in 6hrs.