I think it depends on the "how" and the "why" of her motion sickness. My husband gets very bad motion sickness triggered mostly by 1) not sitting facing forward in motion 2) side to side motion 3) Being unable to see the horizon.
Despite feeling queazy he did manage to survive the Anegada ferry last year on extremely rough seas because it was forward, constant motion and he sat by the window. It was also one trip, followed by a day on land to settle.
However, he did NOT do well going out with Elaine. Her boats are small and you stop quite a bit for snorkeling so the boat rocks constantly. I will never take him on her excursions again.
He has tried everything known to man (including prescriptions, patches, wrist bands, Bonine, dramamine on and on.....) and if it's a trip that is going to make him sick, nothing helps.
If your guest reports that she gets very sick and nothing she tried helps, I would suggest skipping Elaines boat. DiveBVI has a much bigger boat and you can sit up top. My husband, while queasy by the end did manage to survive a day out with them.