This may be harmful to that individual in the long run, so we should be careful about how often and how much we validate such feelings.
All too often do we conflate validation with the latter.
This reaction is generally unhelpful for the listener, as it basically asserts that (a) the listeners feelings don't exist / are not important or (b) that the listener is feeling the wrong emotions.
Validating emotion is "Yes, I understand why you feel that, having <thing> happen to <you/me> would certainly make someone feel <emotion> " e.g. "Yeah, when your friend criticized your cooking I bet that made you feel bad."
After validating the emotion (to create empathy, to create a safe space for conversation, and emphasize that you care about the person), you can then (optionally) suggest a different (more positive) thought e.g. "Perhaps your friend only meant to say that they love using melted butter on their own pasta, and was not trying to say that the pasta was dry." or supply context e.g. "Alice has been having a rough time at work the last few weeks, that might have been why she was short with you."
So validation of emotion is used to create acceptance ("we accept that emotion currently exists and is a reasonable feeling to have in that context") and to create a safe space for talking about emotions in general and the situation in which they arose. (It is especially useful when teaching children about emotions and teaching them to look at a situation objectively.)
Invalid emotions could exist, but usually an over-the-top emotion (and the actions that you take because of that) normally seem to occur because either (a) some sort of miscommunication occurred ("I thought we were talking about X") or that some sort of important context was missing ("You know, Bob lost his job last week, there's a reason he didn't come to the party, and it wasn't to slight you.")
While I can see where analogy of "make invalid states unrepresentable" is coming from, but I feel that how we talk about them can get in the way of changing them. "Emotional invalidation" is a good search term to find phrases that shut down opportunities for change.
or "correct" meaning "you are behaving as society expects you to behave in this situation" ?