There is a way to protect his asset all the way through... the main thing is keep everything separate and be able to transfer and track the funds from the sell of the house, to an individual account in his name, and then to a town home in his name only.  I strongly recommend that he have an attorney review every document in this process to make sure that he keeps the title clearly in his name.  If anything gets muddled in the process, because her name appears on any documents, then she could at least make the argument later that your son gifted her part of his interest--- which means that later, if they file for divorce, he could be required to buy out her interest.