Sorry, I'm staying now just for fun.
You say you like to understand and fix your cars, and that's great. Don't we all. But what's this got to do with your mate getting his car mapped and the tuner not doing a very good job?
Do you think I have limitless funds? I don't I promise you. Which is why I don't waste money, and make sure everything I do is done properly. And I guess I just feel bad for your mate because he's spent money on a remap, when any reputable company would have thoroughly fault checked the cars hardware, and if there were hardware issues notified him before they did the work and probably turned him away and told you to come back when it was ready for further tuning...or fixed it for him.
It's no good doing a job and then saying "oh by the way, you won't be able to make use of it because along the way I found something that was causing an issue but I ignored it."
But if your mate had of read some of the information that I have spent hours of my life writing for the stickies on this wonderful forum that I'm wrecking then he would have fault checked the car thoroughly before presenting it for mapping. And hopefully taken it to a more reputable tuner.
He's paid a company for a service, if the car isn't running right then surely he should be taking the car to the tuner who mapped it and be working with them to sort it out...If not the very first thing I would do is take the car back, say the service they have given is crap and that if they don't refund the software and revert to stock you will report them to Revo.
Then use the money to get the car in a presentable state before taking it to a more reputable tuner.
Anyway, to try and be of some help.
You've worked out that the engine is requesting "stage 2" boost, so now you need to work out why it isn't delivering it.
You need to log the duty cycle of the N75 valve as said, and see if it is functioning correctly.
If it is you need to work out why there is no boost (actuator is the best place to start) or where the boost is being lost (boost leaks, generally everywhere.)
Have you done any form of pressure/smoke test on it?