A single occasional episode like this is bound to happen in most people's life. It may be because of high BP
or low BP
or low blood glucose
levels, electrolyte imbalances. There is nothing to worry as long as the incidence does not recur.
If it recurs often, then only she would require further investigations.
As of now, I agree with her treating doctor that she has to eat and drink properly at regular intervals.
I wish her good health.