Remind her how ballroom dancing is a form of exercise and helps anyone tone up. It may seem obvious that any kind of physical exercise such as dancing will help you lose weight or improve your fitness level. If you and your friend have a good sense of humor, motivate her with the amazing physiques of professional dancers. Print a few photos of a scantily-clad performer and pin them up where she can see them. Occasionally joke with her while pointing at the picture and saying "Do you want abs like this? Did you sign up for that ballroom dancing class?"
Explain how dancing is a bonding experience that helps him grow closer to his significant other. Many forms of ballroom dancing obviously require partners to hold one another close for long periods of time. These actions can help reignite a spark in a relationship that has dulled due to the business of life and kids. If your friend isn't currently married or dating someone special, explain how ballroom dancing could also help him to meet new people. Be ready with a few examples of people you know who met a new dating partner that way.
Rationalize how ballroom dancing allows her to have some time to herself. Executives and stay-at-home parents alike are all looking for more time to themselves and may balk and say they are too busy to start ballroom dancing. Reason with your friend that she is more likely to enjoy some free time if she schedules it. Explain that taking a class is just an hour or two every week or month and that it's healthy for her to make that time for herself. Besides, on the dance floor, she can't be bothered by buzzing cell phones and other modern technology.
Illustrate the examples of how ballroom dancing is trendy and that he will appear cool. Several reality shows pushed classical ballroom dancing into the spotlight and the form of art subsequently gained popularity. For the single person, he can show off his skills and stand out to an potential mate. For someone who's married, his partner will love they are doing something together that, so far, she has only seen on television.