I hope everything works out for the best. This is a tough situation. The fact he is not frum is going to make it difficult to put some teeth into everyone’s suggestion since he and his girlfriend see no barriers prohibiting them as they lead the same secular lifestyle with the same habits.
Ironically, it will be the non-Jewish partner who will have a problem with things, in time, and who will help to teach him a lesson.
Jewish education is imperative. A classic case of an ounce of prevention is worth more than a pound of cure.