Banned from the calculator app

Not if you are using the Apple included app. If there is a message indicating they are banned, they are NOT using the Apple calculator. Have your friend remove the bogus app.

Unless, of course, your friend is lying.

Does your friend have WhatsApp installed on their phone?

I assume you are talking about the Apple Calculator app installed as part of the OS. In that case, your friend is lying to you. Some time ago there were posts on Social Media making that claim, but it was just a hoax to drive traffic to their post.