set phrases - Meaning of "何もそこまで"

ものすごく酸っぱい梅干しを食べたみたいに顏をしかめて、猛烈に残念がった友蔵{ともぞう}に、「何もそこまで」とおかあさんが、みんなの気持ちを代表して、ボンっとつぶやいた。
Mum, representing everyone's feelings, muttered 何もそこまで to Tomozou who was acting as though it (what he'd just heard) was terribly bad luck, and making a face like he'd eaten a really sour umeboshi.

Literally 何もそこまで means "nothing to that extent". So I get the idea that she is saying that Tomzou's reaction is a bit extreme. But what does she actually mean? Is it an instruction telling him to stop taking things too far? Is it just a comment that she thinks his reaction is excessive? Or something else? How should 何もそこまで... be completed?

Answer

This 何も here (which I will call the "protestive 何も") should be differentiated from the 何も meaning "(not) any".

First, the pitch accents are different (at least in standard Japanese): なにも{HLL} ("protestive") vs なにも{LHH} ("(not) any")

Second, the protestive 何も always appears at the beginning of a clause, whereas the (not-)any 何も can appear in various positions.

I labeled this the "protestive '何も'" because it is used when you are protesting against another person's action that you think is extreme, unfair, unreasonable, inappropriate , etc.

Some example sentences to help get a better feel of it:

ちょっとからかっただけで、何も泣くことはないだろう。(It was just a little harmless teasing. Don't cry like a little baby.)

彼は二回遅刻しただけですよね。何もクビにする必要はなかったのでは？ (He was just late for work twice. Did you really need to fire him? (Wasn't that a bit extreme?))

プリンを勝手に食べたのは悪いと思うけど、何もそこまで怒ることないじゃん。 (I'm sorry I ate your pudding, but do you have to be so mad about it?)

As for "何もそこまで" in question, you could say there's omitted material after it. In that case, it would be something to the effect of "何もそこまで(残念がることはないでしょう)", which would translate to "You don't have to look/be so disappointed, don't you think?" But chances are that お母さん had nothing specific that she left unsaid. In context, "何もそこまで", without supplementation, was sufficient to convey what exactly she wanted to say.