Ionic compounds and van der Waals forces
London dispersion forces are instantaneous dipole-induced dipole interactions. All compounds can have an instantaneous dipole moment, including ions, so London dispersion forces exist for all compounds. 

Unlike molecules, ions are charged. The electrostatic interactions between charges are far stronger than the London dispersion forces between ions, so normally dispersion forces are ignored when considering ions. 

Therefore, both your textbook and your professor are correct.