What Causes Sharp Pain In Buttocks With Imbalance?

My friend has been having vague discomfort on his hips and knees for a couple of months now. He ran a battery of tests with a Rheumatologist which included MRIs of his back, and everything came out to be normal. A few days back he started experiencing sharp shooting pain down his left buttock, and fell down in the middle of the street when his left leg gave away. He has a history of a small cold and fever a week back, and nothing else. He's 23 yrs old, and probably underweight for his age. Smokes, and drinks regularly. No other prominent medical or surgical conditions.