I have an 82 year old friend who I help and care for. She has about 10% of her sight left and she uses and needs her phone on a daily basis.

She particularly needs Whatsapp.

I am having a lot of difficulty finding and installing apps with very large clear fonts, in a way where the operating system doesn't keep having overlays or changing things and keeps getting stuck using her phone?

What are the best options?

Doesn't Android have a lot of accessibility options built in? I know Material Design certainly does, but many apps don't use it. Maybe use it in a similar manner to fully blind people.