Luncheon Etiquette

Please help with the proper term for the mother of our son's girlfriend. She will be taking us to lunch and I would like to know how to refer to her. They are not engaged, so
I really don't know how to address her. Also, what is the most polite expression to use when we are first introduced? I would like to tell her how much we appreciate her kindness to our son while he lived in Japan as a student. Thank you so much.

Just adding "san" to her family name is fine, or you could call her "Mrs. (family name)" if you prefer- there is no need for it to be more complicated than that.

I suppose that's what everybody worries about. Luckily, in the Japanese language, it is easy to omit names and references, and people try to get away with not saying any term at all. Or many would just say, "(child's given name)'s okaasan". But I agree with Sira.

Also, why not say, "Please call me ---." and then she'll probably say the same about her. I'm guessing that she is worried about how she can call you, too.

As for typical parents' greetings, you can say,
"Hajimemashite.": Nice to meet you.
"Musuko ga (taihen) osewa ni nattemasu." : I know my son has been in (very) good care thanks to you.

Bon appetite!