My g/frnd is vomiting a lot 5 days after we had sex and she was on periods when we had sex. can morning sickness start that soon? less than a week?

Not likely: The nausea and vomiting associated with pregnancy typically starts about 4 to 6 weeks or later and usually resolves after about 10 or 12 weeks. 5 days after sex would be a bit early for the nausea to be due to pregnancy - but nothing is 100%. A pregnancy test will usually be positive by about 10 days after conception - that will answer this question for sure. Good luck.

Unlikely: It sounds more like a stomach bug. Try to keep her hydrated with frequent sips of liquids ( no milk) and if this persists get her to her doctor or an urgent care clinic.