My friend is a diabetic - recently diagnosed with severe arthritis in left shoulder - now he has numbness / tingling in his right pinky finger and it is cooler than the other fingers - what could be the cause?

His diabetes.

diabetes.

If you take a look at the first link, it will list symptoms that include nerve damage that will culminate in numbness in the extremities.

It is concerning that your friend isn't managing his diabetes effectively.