Can you help Wyatt, a Jack Russell Terrier, with his separation anxiety?

My good friend adopted Wyatt six weeks ago from a canine rescue facility. He is about three years old and does not appear to have been abused. Wyatt is fine when his new owner is home, but when she goes to work, he howls mournfully and incessantly. I go over to walk him during the day, so he is not alone for more than four hours at a time. His constant howling occurs whether or not he is in his crate as she has tested both methods. She has also purchased one of those snug coats that is supposed to be a soothing and calming feeling for dogs. It has no effect. There is also one older dog in the home and two cats, so he is not without company in that respect. My friend is a patient woman and does not want to give up on Wyatt. She is willing to speak to an animal behaviourist, but I thought I'd seek out the opinions of the fine people right here on Yahoo Answers. Thanks for any advice you may have.