How to motivate a person to work out
The only way to motivate others is to demonstrate, by example, the effort, and the reward, of being self-motivated.
Nothing else works. And, sometimes, this doesn't, either.
Bottom line: if a person wants to work out, he/she will. If a person does not want to work out, he/she won't.
There are plenty of examples of both kinds of people, who can be easily known and seen, in every community. The rewards of each type are also easily seen.
The question is, whether the person wants the rewards of working out enough to overcome his comfort zone, which is itself, a reward.