There is a very good book just recently published called “Till Faith do us part” It is about mixed faith couples (not just Catholic ones). The author is Jewish who married a non practicing JW. She has a whole chapter in there about marriages with Muslims and it emphasized again that Islam does not approve of a Muslim woman marrying a non-Muslim which basically makes her an apostate within her family and community even if the family is not serious about the faith. I think the book would be helpful to you. While the book does try to put a more positive spin on mixed faith couples, it does point out that mixed faith marriages are more difficult and less happy and when the parents are at odds or decide to let the children choose, the kids usually pick no faith. While your son and his girlfriend may feel that they are modern and tolerant and free to do whatever, the reality is that when children start, people usually go back to their roots. Likewise, family traditions are very influential even if the couple thinks they are free from all these things. I think this book might be helpful to you but likewise maybe your son might be open to reading because it’s not a “Catholic” book but a very realistic study of different faith couples and their issues. I wish you the best of luck.