What do I do about a mentally ill friend who wont get help?

A friend of mine has recently gone mad. I have no other way of saying it. He has always been quite strange and has severely abused alcohol and drugs. Up until now however he's been fine but recently he has been acting incredibly strangely. He seems to think he has bipolar disorder but refuses to seek help, considering the mania side more of a 'gift' as he puts it. He rambles on about complete nonsense seeming to think he's being profound but makes little sense. He also regularly says quite alarmingly violent things. No one really knows what to do with him, and refuses to go to a doctor, can anyone help?