When Ben realizes he has been eating a cheeseburger, he spits it out, proclaiming that he is kosher. However, if he really kept kosher, he wouldn't have eaten meat in a restaurant that wasn't certified kosher.
Throughout the film, Ben states that he is kosher, but at the Shabbat dinner, his moms serve brisket and announce they have Italian and ranch dressing for the salad. If they kept kosher, they would not serve a dairy item during the same meal as a meat item.