One of my readers wrote to me a few months back as her one cat was extremely distressed when her daughter went to university. The cat howled all night and was inconsolable. I suggested she try a calming spray for cats, which she did, and it worked immediately. From the first use her cat was quiet and slept all night.
I have used Feliway calming sprays and diffusers very successfully in the past when my cat had to go to the vet, when he was anxious and traumatized after being run over by a car and also when we got a kitten. It really worked.
Our local vet also stocks this one