Not all changes are healthy. If the “GF version” involves secrecy, withdrawal from all family and friends, mood swings, or signs of emotional abuse (e.g., fear of upsetting her, isolation from support systems), then concern is warranted. In those cases, focus on keeping communication open and offering unconditional support, not criticism of the relationship.
If they have a disagreement, avoid taking sides or interfering. Encourage them to work it out together. My Sons GF version
Mention how she fits in with the rest of the family or siblings. Not all changes are healthy
If your son and his girlfriend are serious, they might start discussing long-term plans. Be open to listening to their thoughts and feelings about the future. withdrawal from all family and friends