Your friend ia goif through  grieving process. Many people think of grief as a single instance or short time of pain or sadness in response to a loss – like the tears shed at a loved one's funeral. Grieving involves many different emotions, actions, and expressions, all of which help the person come to terms with the loss of a loved one, be it a friend or family member. You can give him your shoulder to lean on, provide different environment that gives him a change of place,  diversion from the grief mood etc. If you still find no improvement, better to take him for a Counselling.