Everything has been in such a state of flux since my dad died that I try my best to ground myself in the truth.
There's nothing much more unsettling than having reality questioned when that's an oxymoron at heart. While it's easier to accept a person holds a different opinion to you, to be an opinion, it has to be a concept you can debate, and not an undeniable event. Having to defend whether a tangible thing happened moves the goalposts, as you're then dealing with a person who's happy to shift the unshiftable to make their point, which means they'll use lies as evidence of their version of the truth.
Half of the problem I've had with a person lately is due to that paradox. While it shouldn't be this easy, blank denial is almost an impervious argument, as you can't get anywhere near to the point. And if someone is prepared to say black is white repeatedly, it's not a conversation; it's a stonewalling exercise.
But if you know someone's main tactic when faced with an uncomfortable truth is to either omit it from other people's knowledge or lie about it, you shouldn't be surprised when they don't adapt. And while it may be an effective way of refusing to discuss the meat of the matter, it's actually a very primitive tactic that fails at the task in hand. For the way to win a debate is to prove your point sufficiently so the other side believes it and not to underline an obvious lie by compounding it with an even bigger one.