can see that your friend is really working with his attempts of maintaining a healthy lifestyle.
in itself can lead to lot of problems though. before we blame stress lets us see whether he has any other symptoms such as weight gain, or loss, pain in abdomen, dry skin
. as he has been seen by a doctor the obvious medical causes such as hypothyroidism
must have been ruled out.
if not associated with any other symptoms other than bodyache we may be relieved that this is not bad though needs attention.
would suggest you to read about irritable bowel syndrome
which occurs as a body's subconscious reaction to stress. treatable though not curable.
hope this helps.