Fortunately or unfortunately, a paleo, low carb diet tends to restrict fertility in ladies because paleo just naturally is more satiating and thus folks eat less food.
This is because of our evolutionary genetic makeup and how our bodys react to the amount of calories consumed. Evolutionary wise, when we eat paleo, our bodys think we are heading into winter time when there is less food available or will be less food available to nuture a fetus to term. Lots of unwanted amenorrhea in the paleo community.
How to fix fertility in paleo females...females have to eat lots of meat and fat with some unprocessed carbs like sweet potatoes and yams and fruits in the late winter..early spring to in effect "fatten up the female body" to ovulate after coming out of what the body considered a winter famine..a dearth of food.
Just suggesting paleo as a fertility remedy may not be fruitful.
As to the appropriateness of your advise, how important is it to the couple to conceive? And how important is it to you to advise your GF to assist this couple? Why do you think you should assist this couple through your GF. If it is important to you, talk directly to the husband.
If you want third party validation, ask Robb Wolf He has talked about paleo and fertility. http://robbwolf.com/2009/12/29/the-paleolithic-solution-episode-8/