Ok. 3.5cm is “medium” on the Spetzler Martin grading but it may be it is in an “eloquent” area of her brain and/or has deep venous drainage (see the link above) which would be good reasons for it being inoperable / very difficult.
It does sound like getting a second opinion would be a good idea. It is fair to say that if she is in danger from her AVM, as you describe, that flying could be dangerous for her, or it might be ok. But you can get surgical second opinions from a number of US neurology institutes without the need to actually travel there. Doing that is affordable but will only give a high-level assessment, I think. Ill find some links for those as well.
Richard