It could have seriously raised her blood sugar. People with high blood sugar are in danger of DKA which certainly can be life threatening. However, DKA is usually caused by being high for a period of time, not just one "spike".
Do instruct your staff to be careful in the future with the diet/regular thing. But since you don't know the whole story, and the girl is ok, don't beat up on yourself. If this is a legal matter, then you want to have the advice of a medical professional not just a peer board.
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