I've had very similar conversations with my mom too. With my mom, I always get annoyed because she asks my opinion and then ignores it. If she didn't want my opinion, why did she bring it up?

Hopefully your mom will decide that she likes the bike and actually starts to use it. As a formerly unfit person, I don't think there's anything you can do to get her going other than be supportive and encouraging. She'll do it if she's ready. Good luck!