I'm also prescribed adderall, both the immediate release to take in the morning and the XR in the afternoon after the tablets wear off. I worked for Red Bull for two years so I've built up quite a tolerance to it; but I digress, it does seem to enhance the effects, not so much the focus aspect of it but the euphoric 'I can do anything' feeling. While this is true for ME, your friend stands correct as Red Bull has (in an 8.3oz can) 160mg of caffeine and a chemical known as glucoronalactone, which is a detoxifying agent of the liver, a carbohydrate that naturally occurs within the body, which as a result seems to sorta 'gobble up' the normally extended effects of the drug alone, but intensifies the initial effect while it does so.
My advice: If your dosage is anything like mine, I've found the best scenario is to take your proper dosage (the salt tablets in the morning) THEN drink your Red Bull about 10-15 minutes before ingesting the XR. You'll experience a longer lasting effect and a bit of a 'peaking' transition as the XR properly absorbs into your system. Keep in mind you have to eat properly so the XR isn't just sitting in your stomach with leftover amphetamine salts and Red Bull; you probably know by now one of the most noticeable effects of the drug is appetite suppression.
My advice version 2: Don't use Red Bull. The detoxifying agents in the drink are designed to more or less defend your body and rid itself of most of adderall's beneficial effects. Try another energy drink like NOS, which in addition to not tasting like bitter horse urine is one of the few drinks in the industry to contain more caffeine than RB, but without the detoxifying 'benefits' in it.
I do hope you're using it simply for studying because it's a dangerously addictive amphetamine, and if you're looking to just peak you might as well join the masses of abusers and just snort the salts, or crush the XR beads into the same compound and do the same. I will add that this is very stupid if done consistantly as I've watched friends fall into horrible withdrawls once their supply has run out.
Ironically I write this in the morning, before I've taken any adderall, I just happen to have seen your post (albeit over a year later) and wanted to give a first-hand answer with a little info about RB. What can I say, I'm passionate about the beverage as it is. I'll check this board in a few days for a response if you haven't discovered a clear answer by now.
Best of luck
AarunJon
drinking just redbull can be unhealthy... if your using 2 stimulants it might not work out very well in the long run