IDK what a point of heroin is ( are you referring to a bag/cap/1/10 gram?) and how good the heroin is, but,if your friend is opioid tolerant and hasn't OD'd within an hour, I think he'll be fine. I used to do massive shots of good heroin and eat double digit mg amounts of Xanax at the same time daily, along with liquor, for a long time and was always good. I have been off heroin for several years now, but I have been prescribed high dosage amounts of Roxicodone and Xanax every day for years and I have always been fine. I'm aware that opioids (especially heroin, but even high dosages of Roxicodone and various pharmaceutical opioids) and benzos is supposed to be the "killer combo" (and have a good many friends die from it, so, before people start jumping on me for posting this, I don't advocate mixing heroin with benzos). I'm stating my own experience with this combo and pointing out that I have personally always been good, and that if he was going to OD, it would have likely happened by the time someone first read your post and commented.
With that said, it's easy to OD with this combo. In the interest of good advice, you need to babysit anyone who seems dangerously over-sedated on this combo, watching their breathing, coloration, level of sedation, coherence, response to stimuli, motor functions, etc...... Heroin and benzos is a pretty heavy poly-drug cocktail that has killed many and, while your free to choose what and how much you do, I would be very careful and I would make people aware of how lethal this mixture can be. If you're going to mix these, I would advise doing the heroin first, letting it peak, then doing the benzos. Lucky your friend has you to babysit him and make sure he's good. Sounds like it will be all good.