Hi. Firstly, it's great that you are thinking so much about your friend, which is rare in today's world. Relationships are the best part about our lives but they are not always easy to manage and when it ends in a breakup, it effects us deeply.  The loss of the relationship is equal to grief, which if  isn't processed properly or you don't get out of it could lead to feelings of emotional void or drained as you mentioned. I would ask you to ask your friend to see a counselor with whom he could process the loss and further find solutions to become emotionally stable like he was. Hope this helps.