He revealed his super-secret patronus that Dumbledore told him to keep secret, a super secret.
Not that it were very important, but actually Harry told Dumbledore to keep the patronus secret, not the other way around.
Also, merely seeing the Patronus isn’t the problem. Understanding it is.
And knowing what it can do—killing a dementor, which they didn’t see, though someone might be able to figure out that his super patronus is the reason why dementors are afraid of him.
Actually, even knowing what it can do doesn’t break the mundane patronus.
Not that it were very important, but actually Harry told Dumbledore to keep the patronus secret, not the other way around.
Also, merely seeing the Patronus isn’t the problem. Understanding it is.
And knowing what it can do—killing a dementor, which they didn’t see, though someone might be able to figure out that his super patronus is the reason why dementors are afraid of him.
Actually, even knowing what it can do doesn’t break the mundane patronus.