punctuation - Proofreading a novel: is it okay to use a question mark with an exclamation mark - "?!"

Is it ever acceptable to use an exclamation mark following a question mark?

I am proofreading a novel and have been instructed to make no stylistic changes, only errors that impede sense/clarity. The copy-editing phase is complete, so if something is acceptable, I must leave it be.

At one point in the novel, one of the characters responds in an incredulous manner to a piece of information:

"Really?!" was her friend's reaction.

I'm not sure how much leeway to give to 'poetic licence'. The style of the novel is very traditional and the use of punctuation is conventional throughout i.e. not attempting any innovative or idiosyncratic use of language.

I know that most style/usage commentators would frown on the use of "?!' in formal contexts, but is it something a writer of fiction can get away with?

Advice from any experienced proofreaders would be much appreciated.

Answer

You have been given a precise task: To correct grammar, not style.

A combination of question and exclamation mark is not a possible stylistic choice but – from the perspective of normative linguistics – an orthographic mistake. In English, a sentence must be terminated by a single punctuation mark.

So if you are asked to correct grammar, you must necessarily mark up this error. As the author, I would expect you to point out to me that this is not correct in standard English and explain to me, when and by whom it is used regardless, empowering me to consciously make an informed decision to either keep the mistake or correct it.

You do not know what the author wants – correct English or creative use of punctuation – so do not make their decision for them!