Brain tumor diagnosis/discussion

So I started seeing this girl, right? Ten days in, she drops the bomb that she’s got a brain tumor and has been grinding through meds for three years straight just to keep it in check. She told me when they first caught it three years ago, it was about 9 millimeters, then it shrunk down to 7 millimeters after two years, and apparently, it’s stayed at 7 millimeters since her last checkup six months ago. I didn't want to grill her too hard since we're still basically strangers, so I just played it cool and moved on from the subject.

But honestly, I’m looking for some insight here—does anyone actually know if it’s even possible to tell if a thing like that is malignant or benign based on how it acts?
I know I'm being vague as hell, but can you even treat a malignant tumor with just pills or whatever...