My boyfriend's mom has a cyst in her brain and it's pushing against it, she's having surgery to remove the fluid, could she die while in surgery?

We can not answer it: While we know there is risk of any surgical procedure and also risk of anesthesia.And rarely can end in mortality.But most surgeries are safe and the only person who can tell you the risks involved is the surgeon who is going to perform the surgery.I siggest you ask these question to the surgeon involved as to the risk of surgery

Risks and benefits: Technically speaking there is a risk of death with any surgery. As noted by others the surgeon should lay out all risk versus benefits.