Your ethical thing is client confidentiality which you must respect, whether you agree or not but at the end of the day its your clients decision, this is not about your personal feelings, its something you learn on the drug rehabilitation side and health side, the side I worked with is was harm limitation to the person so that dangerous behavior was avoided, this was back in the days where HIV was still talked about and its still sad its still a killer today, for most they do move from injecting and risky behavior and most live a longer life until they grow out of the need for drugs.