Excellent piece! My son and his GF broke up and I really liked her. Problem was, she wanted many kids and my son wanted none. I am glad they got that out on the table before wading into marriage or living together. My son is still alone 5 years later.
Realistically nothing is all all you can do. It is her life and your son's life in which you should not take part or have any say unless asked.
Look what happened to Gaby Petito after she moved in with her boyfriend and his parents. I thought it was very strange that she lived with them and not her own parents and will always wonder how things might have turned out if she had stayed in her own home and spent some of the time, not all of the time with her killer and/or his parents. or if she had lived on her own.