Vomit, Diarrhea- unsure cause- best route back to gut health? Best food or supplement for recovery?

[sunny]

My healthy 19 year old had a bout of diarrhea and then vomited once. She has only been sick with vomit maybe 3 other times in her life. Felt better after the vomit. Unsure of cause. She is on holiday break from school, we make our own food, the only outside contact she has had is her father and I coming and going from work. We were not ill. No fever. Now just a lingering feeling of loss of appetite, malaise, general weakness you would feel after a sickness. What would you do for the best route back to gut health? Best foods? Is there an appropriate supplement? Thanks for any insight.