Work politics: Boss’s romantic partner is targeting me — how do I win this game?

I am 27 male, living in Tokyo. I’ve known my current boss for about two years. We first met when I was his personal trainer at a gym, and later, because he can’t speak Japanese and I can, I started helping him with various tasks and errands in Japan. Eventually, I joined his company as basically his assistant — it’s just me and him in the office.
He’s 35, runs a real estate investment/management business (buys property and land, hires property management companies to handle them). I handle all the admin, paperwork, and local coordination. He trusts me and I’m the one who knows how to deal with the Japanese system.
Recently, we went out socially with one of his newer female friends — she’s 41 and has also slept with him. After the outing, she told my boss I was “too young, not mature enough, not detailed enough,” and even complained that I chatted with the driver during the ride because she found it noisy. She told him how to “train/PUA employees” — basically implying he should control me more.
This is the first time I’ve ever had someone talk badly about me behind my back to my boss. In Japan, I know etiquette and norms well, and I’ve never had an issue like this before. I dislike her, and I’m worried she might influence my standing with my boss. She hasn’t known him for long and doesn’t provide him with any real value,
Given this situation, what’s the best way to protect my position and possibly turn things to my advantage?

by RequirementNaive6064

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