victim, bystander or abuser?
victims will absolutely minimize their experience, because denial and not feeling is how you survive, and later on it makes them feel like they’re going crazy because “it wasn’t that bad” only gets you so far when you’ve got massive trauma constantly suggesting that that yes, it was.
abusers will do it because it’s part of the abuse, to get the victims to not trust their instincts, and convince them that nothing is wrong so they’ll stay.
bystanders do it for a ton of reasons, such as because they’re clinging to just world theory (“bad things don’t happen to innocent people, because if it did, bad things might happen to me" or "to be fair, the victim shouldn’t have X") or because they really need to see the abuser as a good person, or because they are pampered, lucky, and speaking from complete ignorance ("twelve is totally old enough to sleep with 35!")
you gotta find out why specifically they’re doing the thing, and tackle that first. so if someone’s a bystander doing the “i don’t get why this is abuse” you would educate them, if it’s an abuser you shoot them in the nuts with a double barrel full of rock-salt, and if it’s an abuse victim, you gently ask them questions or share your own experience, whatever you need to encourage them to feel safe in confronting their own negative feelings.
And as to what to do about it: Depends on who’s doing it. If it’s a victim, it may or may not be a problem, because how much of a big deal it is to you can vary hugely. I mean, some people just plain aren’t that traumatized, and they’re allowed.
In general, if it really is a big deal, and people are downplaying it, well. How important is it to you to change their mind? What do you want to happen? Like, for instance. Do you want someone to change their behavior? Stop being friends with someone? Call the police? Break down crying and become a broken, hollow, shell? These are different goals, and you would use different strategies to achieve them.