[Serious] What’s a good response when someone says so and so is such a nice person and you know they’re anything but?

Nothing. They may be a good friend to that person, even if they were not a good friend to you. You simply don’t know.

If you must say anything, leave it at, “We don’t get on,” and move on from it. Always take the high ground, and let people figure this stuff out for themselves, if indeed, they end up having to.