iPhone Sending Random Text Messages on it’s own.

Tell her to make sure she locks it before putting it in her pocket.

My thought was she was basically pocket dialing her unlocked phone... since you didn't explain the circumstances, there wasn't much else I could safely assume.

Now that you explain it, it sounds much more like someone may have tapped the microphone icon and it was picking up background conversation and transcribing it to text.

Another possibility is the touch-screen is failing.

Smart Phones neither have the intelligence nor has the hostility. Hence we need to look at our settings and the way we synchronise our smart phones with Mac/PC/iCloud and share our valuable data with friends and family members.