One you’ve identified that a person is experiencing a drug overdose, either call 911 or if there is someone close by, ask them to call 911. The most important thing is getting someone naloxone, so prioritize that.
When calling 911, it is often best to avoid mentioning that the individual is using drugs as police may be sent in addition to EMS which can be traumatizing for some individuals and can lead to stigmatized care.
When speaking to the 911 operator, it’s best to say, “My friend is unconscious and is not breathing.” If they ask whether you know what happened, you can say that you aren’t sure or just describe their physical symptoms and ask for care advice.
When EMS arrives make sure to let them know that you have given the person naloxone.