My son was given many gifts by his ex girlfriend and now she is threatening to sue if he doesn’t return them. Can she do that?

Your son's ex can sue him for anything she wants, but if the Court believes that the property was given as a gift (as opposed to a loan or whatever claim she may make) he will not have to return or pay for it.  It is possible, but very unlikely, that the police would arrest him over this type of dispute.   They are likely to tell her, correctly, that it si a matter for the civil courts.  Of course, if she alleges some other crimes, particularly involving violence, things may go differently.