I have such a great time reading all the posts from you seasoned cruise veterans, imagine my excitiment when I could finally help answer a post. As a Nurse and seasickness sufferer I can relate to your friends. My first cruise I spend the first few days a lovely shade of green!
Over the counter treatments such as the ginger and the Bonnine can be very helpful for mild sufferers. However, for some serious sickness they may want to visit their doctors & get an Rx for something stronger. I personally find promethazine (Phenergan) very helpful. My uncle (and physician) often orders Antivert for his patients.
Good luck to them (and to you)