As you say, this is real life. The days of easy mortgages are supposed to be over. They will look for where the downpayment came from. It can't just appear in his bank account one day and be written as a check the next. They will question that. He will have to show that he sold something or that it was gifted to him. And there is a limit on how much a person can gift in a year without the recipient paying tax. So, he will owe tax on the money. I'm sure your sister knows all this stuff better than I do.
