I’m not too keen on the idea of forcing people to recertify and fitness varies greatly according to the person and the type of diving they want to do.
However, this is always a topic that comes up with my dive students. I teach a lot of military people and they are interested in incorporating diving into their fitness routine and vice versa. (Personally, I like to do a little strength training and cardio. I’ve also found that regular yoga practice helps with air consumption.)
I always make sure I talk to them about diving within their limits and if it has been a while, to be up front with their buddy or dive master. Most importantly, I give them permission to be judgemental about the skill/health/sanity of the people they dive with. Their life depends it!