My grandma has an android phone and I'm fine with her having a phone that could permit her to sideload an app. Having a phone number at all is a far more serious threat, and I presume you are fine with your grandmother having one even though a scammer could talk her into giving up her bank details or buying dozens of gift cards and reading out the codes.

The solution to the grandmother scenarios is to have a trusted relative that works closely with them, who they trust to copilot or handle completely all business dealings. If that's not possible, then they're at risk whether they have an iphone or android.