Be in a calm, safe environment and have gentle reassurance that everything will be OK, is the primary treatment.
A large 50-200mg dose of CBD (without significant amounts of THC) can mitigate the effects of THC thus can act as a partial antidote. Lemon oil, found predominantly in the rind and lower amounts in the juice, has also been used historically for this purpose. Grate a tablespoon of lemon zest and chew it up before swallowing.
Most people do not need emergency medical care unless they have pre-existing heart disease or another serious medical condition. For ongoing vomiting and diarrhea, intravenous rehydration may be necessary. Start Low and Go Slow!!!