I couldn't find any similar usage. Could it be a sheer mistake, or is there more to it?
"What will you", which is not idiomatic in English, appears to be an idiosyncracy, or even a catchphrase perhaps, of the character Hercule Poirot. It appears on other occasions:
"Mon ami, what will you? You fix upon me a look of doglike devotion and demand of me a pronouncement a la Sherlock Holmes!" (The ABC Murders) http://www.bookfree.com.cn/yingwen/agch-theabc/008.htm
"Certainly. I write with perfect politeness, tender my regrets, my apologies, explain that I am completely desolated-but what will you? I have retired-I am finished." (Peril At End House) http://www.obooksbooks.com/2015/3806.html
"What will you? When one is unique, one knows it! And others share that opinion - even, if I mistake not, Miss Mary Marvell." (Poirot Investigates) http://www.obooksbooks.com/2015/4077.html
I suspect it's an overly-literal translation of a French phrase which has a meaning similar to "What of it?", or "What do you want me to do about it?". Its usage in context appears similar to those phrases, anyway.