I can only speak for what I have seen with my husband. I am the one that has labled our friend as his NSP or NS G/F, he just calls her a close friend, but he doesn't even have this close of a relationship with anyone else. Personally I like the term Partner vs G/F because it is a better description of their relationship.
1. If anything happened to me or the kids, she would be the first person he would call. He might even call her before calling me, depending on the situation. Truth is I would also call her if anything happened to him or the kids before even calling my mom and have.
2. She knows more about our family life than anyone else not living with us.
3. People mistake her for his wife and are surprised when I finally show my face. It is actually the ongoing joke that she is his second wife, even from her husband.
4. They can't go more than a day or two without talking, not sure who actually needs to talk more(probably her since my husband is happy just being silent). Texting is another story
5. I don't even blink an eye if she is at our house when I get home from work on his day off.
For the most part, I have never been bothered by their relationship, except when things start getting bad between us, then all my insecurities come to the forefront. They do alot of service projects together and it is nice knowing that someone will be looking after him when I can't be there (or don't want to), she is a detail person like me where he sees the starting line and the finish line and can't see any of the pitfalls inbetween.