I would suggest encouraging your daughters not to worry so much about what potential dates are or aren't eating. If they concentrate too much on this one aspect they may miss out on a great emotional and mental connection. They are young and probably don't understand just how vital a mental connection/attraction is to a long lasting relationship. Obviously physical health is very important, but most people their ages are not so preoccupied with their health. Meeting someone who doesn't share their health values doesn't mean it has to be a rescue mission, it can be a natural, organic process. Once they have a serious potential mate, I'd be surprised if they weren't amenable to moving their diet and lifestyle patterns towards the paleo. I'm assuming when their Dad was their age, he was not a perfect example of health ;) Most of us weren't and you can't blame people for being uneducated about their health in this country. The paleo movement is still very fringe.
When I met my partner he was vegan and drank excessively. He was also extremely intelligent, creative, funny and as kind as people come with a heart of gold. Luckily I de-veganized him rather quickly (just wave bacon under their nose) and now he in better shape than most people our age, his health problems from being vegan are gone and he is just as interested in paleo lifestyle as I am. Never in the years we've been together did I feel like he was a rescue mission, it all happened naturally b/c we really loved each other. You could even argue that it adds something special to a relationship to go through such a change together. My advice for your daughters is not to focus on this one aspect of a potential partner, it's not healthy and it's not realistic. Relationships are hard and require work, I understand they don't find it appealing to have to "teach" potential mates how to be healthy, but if it's not that it will be something else. They might have to teach them how to have a sense of humor in bad situations, or how to express their emotions better, how to be a better listener, how to relax, or how to be more responsible. There is a whole slew of issues like this that come up in a relationship, and you can't always be forewarned of these things from the first few dates. This is a good opportunity to teach your daughters that there is no such thing as a "perfect" mate and finding someone who respects and loves them is priority. Most couple's lifestyles eventually synch up, don't worry about it.