What advice would you give to a person who wants their significant other to start exercising?

I agree with LaRue and Sara

Acceptance is important. You can’t make anyone, a significant other or not, exercise because you think they need to
Support is vital- Encourage and suggest but don’t force
Living the example is a must

Find a happy meeting ground: possibly find an activity that you both enjoy, it doesn’t have to be exercise per se but maybe a dance class that you can both have fun in and participate in together?