A friend of mine can no longer take care of her dog, so I am adopting him. I need some advise!

My friend just moved for a new job which requires her to travel a lot. Boarding the dog costs her a pretty penny, and she can no longer afford to keep him. Frankie (the dog) is a 2 year old rescue dog with some pug, Chihuahua, lab and dachshund (maybe more.) He is very calm around people, especially his "master," and loves to play.

Frankie has a bit of a problem with separation anxiety. He becomes very upset when his "master" has to leave. This usually subsides after 20-30 minutes, but this is only when someone else is there to keep him company.

Here are my questions: What can I do to comfort Frankie through this transition? How can I train him to be alright with being alone?
Are there any issues I should be aware of with dogs that go through this kind of change? Any other concerns...

Any advise that you might have to offer would be fantastic. Thanks!

TL;DR: Adopting a dog from a friend with separation anxiety and need advise.