I work in a office with a non-Jew who told me an halacha she heard from a Jew at her previous place of employment. If that halacha is not accurate, can I tell her that it’s not true, or am I showing disrespect to that other Jew in from of a non-Jew by doing that?
You can tell her, if you think there is there is value in so doing, that the halacha is not true, but ensure you do so in a respectful way.
For instance, you can tell her that with all due respect (etc.), the halacha might be a little more complex than the person in question thought (etc.), and therefore it isn’t quite as she thinks.
In general, I imagine there won’t be much value in pointing out the mistake, but this will depend of course on circumstances.