In real human conversation, when someone is expressing an emotion they aren't looking for other people to confirm that they are indeed experiencing that emotion. That's not even a question up for debate. They're looking for people to share in that anger, sadness, or frustration and confirm that it's a valid response to the situation.
The overly academic definition doesn't reflect how people communicate in the real world.
There's also a factor of consistency over time: It's no big deal to go along with someone venting from time to time, but when someone you're close to is overreacting to everything and having unreasonable emotional reactions all the time, validating those emotions consistently is going to be viewed as an implicit endorsement.
> It sounds perhaps like your family member's former partner was going further than validating the emotions, and trying to justify or prove them right.
Not in this case. Just going along with it.