Not directly with Chiari malformation but with a somewhat related issue, spinal stenosis in a friend of mine.
In doing research about spinal stenosis, I discovered that there is now a type of minimally invasive “keyhole” surgery which is miles ahead of what had traditionally been done for years.
Evidently there is also a similar less invasive surgery for Chiari malformation as well but not a plethora of people doing it.
However, if it were me, I would certainly do everything I could to find the nearest qualified person to do the significantly less invasive procedure.
I have no idea where your friend lives, but if you research the term “minimally invasive neutosurgery” it might give them another option to standard surgery.