The obvious answer is to encourage your friend to stop IVing drugs. The fact that your friend seems committed to continue this behavior given the fact that you describe it in the present tense suggests that he will continue to do so, even though it is obviously extremely distructive to his health.
Many of us who communicate on this board engaged in unhealthy heart practices, but changed behaviors at the encouragement of family, friends and doctors. Most doctors will refuse to treat patients who continue with destructive heart practices. Perhaps your friend needs to post him/herself after commiting to a healthier lifestyle.
Drugs such as heroin will destroy the heart valves, attack the heart muscle, collapse veins and a whole variety of things. This is just the heart, the liver and other major organs are affected too. If the addiction to drugs is strong, then rehab and councelling is the best avenue. Drug users live in denial (it won't happen to me) and sometimes need a wake up call in education. Rehab make coming off the drugs easier because substitutes are given to ease the withdrawal symptoms, which in themselves would otherwise cause life threatening issues.