Need something achievable for an extremely tech illiterate person on iOS. I got told about this after they are already gone and have no access to the device, but am asked to solve the problem.
EDIT: (SOLVED) Signal was super easy, works awesome, and was easy even for my mom to figure out on her own. Thanks
I would say Signal, like others have pointed out.
Alternatively you could try Threema
Second for Threema.