I'll chime in before everyone starts ranting 'Autometer is teh suck' and 'JDM is teh bett4r.'
Autometer gauges are known to be a bit inaccurate as they can leak under high boost. This generally results in the boost readings being off by a pound or two - not the behavior that you're seeing. I have an Autometer gauge and, while not the best, it works for me.
It sounds like what you are seeing is close to the normal boost curve of the WRX. On a stock car the boost will make it up to around 14.5 PSI in 3rd gear. As RPMs climb after peak boost the wastegate will open and boost will taper off toward red-line.
I do find it odd that you are seeing the boost drop and then spike again. Before you blame the gauge make sure you tell your friend to keep his foot to the floor all the way to redline. If he lets off at all during the run the boost will drop and then build again.
Once you have everything tight and you run with a consistent throttle position you should find that the boost hits a peak around 14.5 and then tapers off to redline (peak boost will be lower in first and second gears).