But she posed for the picture, so it was also consensual.
Additionally, the photo belongs to the restaurant owner.
Moreover, given the fact that restaurants have the right to display even indecent photographs, he has the right to display an innocuous one.
It can hardly be considered advertising since the viewers are already customers or prospective customers - they are inside the restaurant. It is not being used as an endorsement or a solicitation.
What this is is a man who has powerful friends and who wields enough influence to ruin this man's business threatening a man who has done nothing wrong.
The letter specifies that it's in the front window.