What girlfriend?
Did anyone in his family ever see this phantom “girlfriend”?
Not every family in America is connected enough that, if somebody was in a relationship, the rest of the family would be pressuring to meet that other significant other.
That does not mean that everybody who declines to show their boyfriend or girlfriend is guilty of keeping them in the basement and abusing them for years on end.
Not everyone brings a girlfriend/boyfriend around their family.
We maybe assuming that this is a close family. I have a brother that I see maybe once every five years. If he brought a child and said it was his daughter I can’t say I would call the police. She very well could be.