What to refer to friends mom?

Here is sort of a random question but, what to refer to my close friend's mum as? Last name is Kozu. Should I just call her Kozu-san? Refering to her as "okaasan" definitely feels wrong to me lol, as I reserve the name for my own mom. But idk if it's wrong or right to use first name in this situation.

ask your close friend.
I would just use last name

Call her obasan おばさん, which means auntie. Make sure it does not sound like obaasan おばあさん, which would mean grandma.

Assuming you are an adult, and not a low-teen or younger, if I were you I'd just get away without calling her at all. That's probably the most common way in Japanese language/culture.

I also often refer to them as "okaasama". What I mean is that if I am not talking with her, and suddenly need to speak to her, I'd just call out "sumimasen". But if I need to say the "you" term, I'd use "okaasama" such as in "AA-chan wa okashi yorimo kudamono ga suki yo ne. Okaasama wa dochira ga osukidesuka?"

If you're much younger, perhaps you can say "AA-chan no okaasama".

Meanwhile, "obasan" is tricky, because those who aren't used to be called "obasan" (for example, those who are more used to being called "obasama") might have a bad impression about it.

Of course, if I were talking in English, I'd just call them "Ms. (family name)".

Hope it helps.