The issur is on the treif cooking, not the “treif” cookbooks. Cookbooks are not treif unless the pages are made from pigskin (and even that is technically not tarfus, but lo tehora).
As a cookbook is not ra-ui l’achila, it does not even have a status of kashrus. I will however, agree with you that given the glut of KOSHER gourmet cookbooks available, there is really no compelling reason to b’davka buy one with questionable recipes. But that certainly does not reflect on your friend’s kashrus. Let’s all find another chynik to hok.