Fear of Heeling
A little understanding of ballast and sail control can help this situation. What helped me when I first learned to sail (and girlfriends that I have taken out to sail since) is learning that there is a keel with a significant amount of lead in it under the boat. This is not always easy to understand so a verbal description to help them visualize the "fin" underneath is helpful, as well as how many pounds of lead it contains. When I told my "most fearful friend" that there were 5000 pounds of lead underneath us it helped to quell her fears. Also, demonstrating to them how some adjustments on the mainsheet/traveler can help ease the situation... (involve them in these controls if they are willing and able) will give them confidence that the boat is not out of control. Of course the most obvious is to stay ashore in questionable weather and provide a life jacket option to those who are uncomfortable.