The best way to motivate a friend to workout is to help your friend find a workout routine she enjoys. If your friend currently lacks motivation to workout, chances are that she hates working out, likely because she finds working out to be painfully boring. If your friend enjoyed working out, she wouldn’t need you to motivate her to do it.
So when trying to answer the question of how to motivate your friend to workout, focus on enjoyment rather than motivation. If you can help your friend find a workout routine he enjoys, that will take care of the motivation. People don’t need motivation to do things they enjoy. People look forward to doing things they enjoy.
If your friend is like pretty much every other person on the planet, chances are she doesn’t like going to the gym because working out at the gym is boring as hell. So encourage your friend to try a group fitness program such as a boot camp, or circuit training workout, or spinning, or dance class, or some other activity or sport that’s fun and also a great workout.
Whatever you do, don’t bother trying to motivate your friend to do traditional gym workouts if your friend hates traditional gym workouts. That’s a lost cause. You might be able to motivate him to workout at the gym for a week, or a month, but eventually he’ll stop because deep down he hates doing those kind of workouts. Instead help him find a workout he likes and will stick with long term.