There is only one answer in the interest of HR.
Don't try dope.
This is true.
However, chances are he would be ok. Now, if he had no opiate tolerance and was planning to bang a bag, I would surely say NO BENZOS INCLUDED, what so ever. This is how naive people die. Benzos and opiates are a risky mix, but if you know what you're doing and you know your tolerance/body, it is a very, very euphoric high. One of the best. Throw a little weed in the mix and it's orgasmic.
If I were you, I'd tell him to do the dope well after he's peaked from the xanax, like 6 hours or so after his last dose. Xanax is a fast acting benzo with a relatively short half life. It has a very quick onset and a short duration in comparison to other benzos, but it's also one of the more potent ones.
Does your friend have any opiate history/tolerance? If so, what has he taken before? I would have him start with no more than a bag (.1) of dope. Depending on how good it is, it may be all he needs to be floored. Even though snorting has a much lower BA (Bio Availability) than the preferred route of us dope users(IV) it can still hit hard for someone who is new to the game. Break the bag out into two lines, and have him start with one. It may be a small line, sure, but potency varies and if you get fire dope, then it'll surely be enough for him to feel it assuming he's new to opiates. Take one line, and wait 15m to gauge where he's at. If he wants to be a little higher, take the other line.
It's understandable that he cannot miss a xanax dose, but if he doses twice a day(every 12 hours) then I strongly recommend he does the dope somewhere in between that time frame, and definitely not at the same time. Chances are he'd be ok, but anything can happen, espiecally when you mix new users with very good smack.