Someone I know shoots up soboxone, or bup and he's addicted to it. How can he get off of it?

Go to Pain Managemen. Very few physicians are authorized to prescribe and deal with suboxone. Recommend to go to a pain management specialist or addiction specialist certified to prescribe suboxone.
Get Help. Obviously you are concerned as you should be. Your friend needs help. Unitil they realize the possible harm that they could be causing, they are not going to change. You need to get those who are affected by this addicitive behavior together and do an intervention preferably with professional help. Many resources are available for those that are willing to accept it and participate.
Addictionologist. Your friend needs to see an addictionologist. Buprenorphine needs to be tapered off and it's withdrawal managed. He/ she may need to be put on an antagonist for a period of time while recovery work is done.
Addiction. He should get an evaluation from a physician who is certified in addiction medicine by the american society of addiction medicine, and follow the treatment recommendations or he is likely to die of addiction. Http://www. Asam. Org/.
Suboxone Detox. Suboxone is, perhaps, even more addictive than heroin or prescription pain meds. It also has greater binding to opiate receptors, making "cold turkey" detox so difficult that most patients relapse before completion. Anesthesia assisted rapid opiate detox is a safe alternative to withdrawal that detoxes patients painlessly under deep sedation in hours as opposed to days.
See addiction MD. He needs to be seen by an addiction specialist. Likely needs inpatient treatment and therapy.