I honestly don't think she actually altered his brain structure at all...it's probably ~70% scaring the shit out of him with the lead up making him think she actually cut his head open an messed with his brain and ~15% each from a dose of something to make him happy for a few days and repeated hypnotic suggestions after he passed out
I'm not an expert but my mom was a nurse who didn't know how to shut up and liked to talk about work whether we wanted to hear about it or not...and I know a thing or two from my own prior (admittedly not brain related) surgeries:
1 - if she actually did surgery she'd want a sterile environment and need tools she didn't appear to have with her (I suppose magic could compensate, but as far as we know now she can't use magic, and I didn't see any known magic users following her either)
2 - to change his personality with any degree of control, in addition to the tools she doesn't have, she'd also need to know how his brain connections map out...the general regions of the brain match up, but specific connection paths and memory related stuff grows in uniquely as we learn and remember and thus varies from person to person (seriously go watch some videos of brain scans while people react to stuff, the same stimuli will make different parts of the brain work at different rates in different people)
3 - from what I have heard (and, ok, admittedly this part I'm least sure about, but it feels like it has a ring of truth to it...) it's a good idea to keep the person awake if possible during brain related surgeries to be sure you aren't *accidentally* messing up connections between anything really important, like the parts that remind you to breathe automatically or how to move your arms/legs or speak...sure, it wont guarantee you wont mess something up, but it makes it easier to notice something is wrong so you can stop and minimize the damage (at the very least, I know my doctors have preferred to keep me awake during...6 total surgeries, 4 I know I was conscious through, 1 I might have been but don't specifically remember due to being in severe shock at the time, and 1 for certain where I was out during but ONLY because I was a kid and kept reacting too much while I was awake so they HAD to put me to sleep to finish)