Deactivating a certificate will disable all the apps that certificate pertains to.
Necessity is relative. If she visits all sorts of random websites all the time, checks out every QR code she sees, and downloads apps from random websites, then anti-virus might be useful. I don't know that it'd be necessary, though.
The reason there's no antivirus for iOS devices is that Apple won't permit it, since an antivirus app, by definition, has to have complete access to all the memory and storage on the device, and Apple prevents apps from accessing data in other app locations. Not because it's less vulnerable - in fact jailbreaking requires a hole or exploit, so we know there are cracks in iOS.