is there a securty app for i phones

Unless her phone is jailbroken, it cannot be hacked. A mail account can be hacked, but not on the phone itself, other accounts such as Facebook or other apps can be hacked at their source, and Apple ID's can be hacked, but not the phone itself. The only way is if someone had the phone in their physical possession for a period of time and could have jailbroken it.

send a message supposedly from her and not physically have her phone.

This suggests address spoofing and not necessarily a hack.

Another device that is jailbroken may be able to show her address as the "sender" when she actually isn't.

What kind of security app? The iPhone has some of the strongest security of any mobile phone.