Solution 1:

How dare he tell her!

We can’t tell because this isn’t a real example with actual surrounding context taken from published literature of some form. I can tell you that in my own idiolect, you can never put the exclamation of indignation (which is what you seem to be referencing here) into the past tense. It doesn’t work that way. It’s not inflected at all, or else it breaks it. Modals are like that, you know. How dare he tell her! stays that way. You don’t do anything to the verb dare.

If you want something to carry tense, you’d have to do it as you would other modals, by using a perfect construction, perhaps like How dare he have told her! But that sounds pretty funny, too, so I would just leave it alone. The context would make it clear.

So perhaps this isn’t the exclamation How dare you! at all, since that wouldn’t make sense in the past tense. It probably isn’t an exclamation at all, since there’s no exclamation mark. But we can’t say for sure because it’s an artificial example. It could simply be a real question asking how it is that you managed to pluck up the courage to tell him.

As to why your googling seemed to miss out on normal past-tense uses historically, there’s a good reason for that...

How durst you!

It’s of course because the past tense of dare is durst! Quoth the OED:

Pronunciation: Brit. /dɛː/, U.S. /dɛ(ə)r/
Inflections: Past tense durst /dɜːst/, dared /dɛərd/; past participle dared;

It even takes the normal contracted forms like durstn’t. For example, from The Lord of the Rings we have that form appear in this dialogue in the “Strider” chapter:

Sam and Merry got up and walked away from the fire. Frodo and Pippin remained seated in silence. Strider was watching the moonlight on the hill intently. All seemed quiet and still, but Frodo felt a cold dread creeping over his heart, now that Strider was no longer speaking. He huddled closer to the fire. At that moment Sam came running back from the edge of the dell.

‘I don’t know what it is,’ he said, ‘but I suddenly felt afraid. I durstn’t go outside this dell for any money; I felt that something was creeping up the slope.’

‘Did you see anything?’ asked Frodo, springing to his feet.

‘No, sir. I saw nothing, but I didn’t stop to look.’

There is no shortage of how durst you specimina for your examination. Historically how durst thou was used when we still had a second person singular pronoun and inflections.

Shakespeare uses it all over the place. Look in the Henry VI plays especially. Here from Part III:

Ha! durst the traitor breathe out so proud words?
Well I will arm me, being thus forewarn’d:
They shall have wars and pay for their presumption.
But say, is Warwick friends with Margaret?

Or here from All’s Well That Ends Well:

It shall be so: I’ll send her to my house,
Acquaint my mother with my hate to her,
And wherefore I am fled; write to the king
That which I durst not speak; his present gift
Shall furnish me to those Italian fields,
Where noble fellows strike: war is no strife
To the dark house and the detested wife.

You’ll find this used as a past subjunctive or conditional form equivalent to would dare.

My mother does not drink wine and my father durstn’t.

Into the Twenty-First Century!

The modern non-regional/rustic/literary past tense form is of course dared, as in:

• How dared you say such a thing to her!

• I dared not say a word.

Just like

• How could you say such a thing to her!

• I could not say a word.

In the present, it’s still a proper modal in negative and interrogative contexts:

• He dare not open his mouth.

Which is very different from:

• He does not dare to open his mouth.

But we cover that elsewhere.

Solution 2:

Q: Shouldn’t the past form of the idiom How dare [you]! be How dared [you]! rather than How did [you] dare!?

A: Let’s back up . . .

As an idiom, How dare [you]! is a fixed expression, a set phrase.

It doesn’t have a past version. Or, rather, the usage of a past version is considered an oddity.

Linguist Bryan Garner, author of Garner’s Modern English Usage, calls it an exclamatory construction involving an inversion:

How dare he do that is an idiomatic phrasing of the interrogative How [does/did he] dare [to] do that?

He goes on to refer the reader to the section on the past tense dare, where he wraps up with:

It is odd, however, to see the past-tense form in the set phrase how dare you . . . Most writers and editors would insist on making [those examples] How dare you!
_{Source: Garner’s Modern English Usage}

Sure, you can do it, but you’ll be a little lonely:

_{A usage comparison of how dare *, how dared *, and how did * dare. Source (enlarged and interactive): Google Books Ngram Viewer}

The Corpus of Contemporary American English shows how dare PRON at a frequency of 3699 compared to how dared PRON at 10 and how did PRON dare at 7.