First, Be glad they called you and told you. Someone told me once at a pre-school or daycare, the kid got someone else's milk and seemed fine, so they just kept giving her milk day after day, never told the parents.
Anyhow I would give it until tom. to see how she is. Also it was just 1 sip, so may not really affect her to see a big change.
I don't know your history, but is she on rice milk because of an allergy or intolerance? How long has she been on rice milk and are you also not giving her any dairy at all, even hidden???
My first son was on rice milk and no dairy at all (even hidden) from 13/14 months to 18 months old.
After his body had time to heal and I guess he grew out of his intolerance he was fine with any type of milk.
He was on in for a major intolerance, not an actual allergy though.