thanks for your query.
is ABSOLUTELY essential during the first trimester of pregnancy
She should not miss folic acid tablets at all.
Nausea and vomiting
of pregnancy could be due to many causes such as folic acid and vitamin B complex deficiency, pregnancy related hormones which cause nausea, liver derangement, gastroenteritis
You should start taking folic acid and ask your obstetrician to prescribe additional medication for the nausea and vomiting, which should mostly reduce by the end of 12 weeks.