What do you mean by "simple"? If you need a messenger with a simple interface then I recommend you install watsapp. This application has very convenient buttons and it is very convenient for older people to call and send messages. In addition, watsap has a lot of features that make our life easier. To make these features available you need to install fmwhatsapp updates
I have been using messengers for quite some time and there are also messengers that everyone can use. I think that your grandmother can choose any messenger for herself.