Naloxone I believe is available over the counter in most states and I don't think you have to be a health care professional. What you are describing is cyanosis. Because of lack of oxygen in the blood the lips and mucus membranes and fingertips may appear blue. This is a medical emergency. A person should be taking 12-20 breaths/minute and when at rest. When this drops down to 8 or 9 breaths this is serious. Resting heart rate is 60-100 bpm but more realistically 70-85. If this drops below this is also signs of a medical emergency.
Try to persuade your friend when he's had enough- a kind of buddy system. Also, speak to your local pharmacist about carrying naloxone (Narcan) if legally allowed in your territory. You may save his life. Yes, smack him in the face, get him to move, crawl, stand, sing, whatever; this increases respiration and blood flow.
I find nodding off completely is a waste cuz you just sleep and don't get to enjoy your high. I've nearly killed myself with alcohol, but I have half a brain when it comes to opioid usage and would not let myself get into that situation. I've known at least 10 people who have died from opioid overdosing and it is really sad.