textual proof of Miss Grey's character apart from what Willoughby tells Elinor?

I can't remember being told any such thing by the ON.

All one can conclude from the non -Willoughby utterances is that she is an older woman, very rich who would like to marry a charming young Willoughby.
Charming and persuasive he must have been.

So my visual image is that Willoughby is posing as a very ardent, charming lover.
Then, Miss Grey discovers about Marianne at the ball.
Willoughby tries to convince her Marianne meant nothing.
To prove this he writes that letter in full knowledge of Sophia.
Perhaps Sophia even approves of/suggested those sort of kind words. Why should we imagine her to be hateful towards one she considers a loser only because Willoughby tells Elinor of her jealousy.

What makes me wonder is that Willoughby doesn't bring up the letter himself.

He wanted to leave;
" "Well, let me make haste and be gone. Your sister is certainly better, certainly out of danger."

But Elinor stops him with;
"But the letter, Mr. Willoughby, your own letter; have you anything to say about that?"

Caught thus he hastens to make light of it by being sarcastic;

..... what do you think of my wife's style of letter-writing? -- delicate, tender, truly feminine -- was it not?"