This might be one of the more loaded questions out there…
What kind of photography does she want to do? Does she want to make prints? Does she want to only upload to social media? Wildlife? Street? Does she want 4K and video compatibility? Is she wanting to a photography business? Does she have experience shooting with a dslr or mirrorless style camera? Meaning, does she understand aperture and shutter priority and can she shoot manual?
Sony was the first to get into the mirrorless game and they revolutionized cameras with their advances. Nikon and Cannon are finally catching up and leaving the bulky DSLRs behind.
There are basically two lines within Sony: full frame and APSC (crop) sensor. Full frames include the A7 (1-3 and R and S) series, as well as the A9 and A1. The crop sensor series is the A6000-A6600. Each one has their pros and cons. Some are great for high MP images and making huge advertising prints. Some are great for videos. Some are good all arounders. The a6xxx series is great for travel, cause it's a power-packed camera in a small body.
Sony's "best camera" costs upwards to $7k (A1). But that's for serious professionals who make a pretty good living off of their work (think NatGeo, high end wedding/fashion/celeb photographers, etc). And that's before you even get into lenses. And it doesn't matter how expensive the camera body is, if you use bad lenses the image quality is going to be trash. Native Sony lenses are ideal (not necessarily the GM lenses), but sigma makes great alternatives for 2/3 the price. Tamron is fine, too.
Personally, I got started on a cropped body Sony camera with a quality 35mm lens (50mm equivalent on a full-frame) and it was a great place to start. I shoot on a full frame Sony now (not an a1 lol) and it's a great piece of equipment.