You can run the android version or use the Linux VM. Neither are great but are workable. Unless they've changed it recently, you can also dual boot them and run Linux off an external drive.

I'd honestly say skip the Chromebook, get an older used laptop that is known to be fully supported by Linux, install a lightweight distro, and off you go if all Grandma needs is a web browser. Older used laptops are usually far better powered than a cheap Chromebook for the same price anyways. Plus it fights e-waste.

A further option is to do adblocking at the router or through the computer's own networking system or something like a Pihole. These all come with their own pros and cons.