There are far too many factors to give a straight answer.
Things to consider include:
will he be supporting this woman financially?
will she be treating the property as her home and doing the sort of things a homeowner does, such as choosing the decorating, furniture and so on?
will she be contributing to the financial running of the home - paying towards the mortgage and/or bills?
will he be taking her income into account when he decides on the size of mortgage he wants (if applicable)
what is he telling her?
In all honesty, if he is worrying about this maybe he shouldn't be moving in with her.