I did the same as your son! My DH had been part of my extended social group for some years, but our courtship lasted 6 weeks. I didn't live at home so my parents had never met him, I do not think they even knew we were going out, just knew that I sailed with him. I just rang my parents up and asked if I could bring him with me when I came home for the day the following weekend and then announced that we were engaged just before lunch.
I seem to remember that my parents took it in their stride. Delighted for us both, dug out a bottle of wine and the day went as normal. In the long term: my parents loved and respected him and turned to him for advice where they knew he could give it. In return he was always at ease with them, willing to help when needed.
So, petitpois, I would just be welcoming and delighted to meet her and congratulate them - as if you had known her all your life.