Unless the timing was right after she was dropped off, and someone rang the doorbell. She might think that it was her friend needing to say something to her. I could see myself automatically opening the door in a situation like this. I would not open the door in other situations. I just hope that they have lots of security cameras, inside and out, especially since the mother lives alone in a secluded place.
Although it is a gated, secure neighborhood, there are always ways to get into the community. If Mom has hired help, they may come in on a daily basis. Who knows if someone has a badge of hers or a sticker on their vehicle...That's how we can get into our subdivision. Visitors stop at the guard shack.
I just hope that she is found soon. I also suspect that it is someone that she knows or it is an associate of someone she knows.
JMO.