That's a common problem with backside because so many people teach you to cheat the spin and start turning your shoulders and looking back at the takeoff before you pop. I struggle with it like crazy.
The trick to that 'cheat' method is you don't look back until after you pop, otherwise you can't time it properly. Your upper body rotates, but your head only rotates a little as it looks down at the lip and you time your carve and release properly. Only once you've popped off the lip does your head look backside into the spin.
I'd actually wager your friend uses the same 'cheat method' but he's just become experienced and good enough that he doesn't need to lead his spin as much. At the end of the day as long as your upper body is leading the spin, it doesn't really matter much if it's by a 5 degrees or 90 degrees ahead of the lower body.
Basically the upper body is just there to tell the power where to go, so that separation becomes less important as long as you are leading with it by that tiny split second.
in all honesty, is this a real problem? As long as I focus on popping off and get a bit more speed to compensate, who cares, right? With this extra windup I feel like I can get around to throw 9's if I can spot the landing in time.
Honestly you can just use more speed to compensate, it's kind of ugly to watch, but I do know a couple guys who do this at first with new spins when they want to squeeze that extra rotation out.
As far as fixing the original issue though, it seems to me like you have two solutions.
1) Carve harder and come in with more angle on your final carve to get your extra power from your carve while using the same release you use for 360s (I assume your 3s release at the right time)
2) Start your release a little later
It seems like you already said you know you release too early, so why don't you just start your release later? The whole 'start releasing as you start to ride up the ramp' timing is just a guide, if you find yourself using that timing, but releasing too early still, then just push your release point a little later by starting everything that tiny bit later.