This doesn't cut it: "We encourage people to actively log in and use Instagram once they create an account."
How long can I go idle before Instagram takes my account back?
Also, as someone else stated in this thread the very least Instagram could have done was to email her and inform her that she was about to lose her account due to inactivity.
Lastly, it's important to note that Instagram didn't just "prune" her account, they renamed it and gave her original account name to an employee. If they were concerned with squatters or dormant accounts they would have actually nixed the account, not renamed it to something else.