A *****ulent transaction happened to my Sister's account yesterday

A *****ulent transaction happened to my Sister's account yesterday. Her account and card was blocked. She logged a complaint with Standard Bank and an investigation is currently happening. My sons account (Standard Bank Account) was stopped because of 1 transaction linked to my Sister's account. We found that information when he called the bank asking why his account was blocked due to my Sister's account. How is it that his account can't be unblocked? He can't access his money for transport for work or anything else? Sure this has to do with my sister not my son's account.