Zhihu, is it illegal to ask mistress for money in private?

It's not illegal. The original match was to ask mistress for money, but to get back the money her husband gave mistress. The money belongs to the original match and her husband's property, and Xiaosan must return the original match and her husband's property, otherwise Xiaosan will violate the law, and it is reasonable and legal for the original match to ask Xiaosan for money. If Xiao San doesn't give it, the original match can sue Xiao San and let the law make a fair judgment.