Agree everyone needs a hobby, if your life revolves 100% around kids and work you will be bored when the kids get older. Wife has noticed this with her mom, her life revolved around kids and had no friends other than her sisters, now that she is older she has no hobbies or friends (other than her kids and sisters).

My wife felt like this about 6 months ago after our 2nd kid, she wasn't getting out enough and becoming an isolated homebody, she didn't want to become her mom. She recognized it so she made it a new years resolution to get out more. I do my best to help, I never deny her a chance to get out for a dinner with friends, I will re-arrange my schedule or skip a workout so she can go to the climbing gym. I push her to try new things and take classes (pure barre, yoga, etc.) I also make sure she knows money isn't an issue in her hobbies.

To answer your question, its hard to bring it up, they need to recognize it. I'd be supportive in anything she does, hint going to a happy hour with friends or co-workers. Maybe bring up getting together with old friends, offer to watch the kids or take them away for a day or something.