That exact thing happened to an i3 owner, in Northern California I think. REx engine failed with car still under warranty, Dealer hooked it up to the BMW service diagnostics computer, and it showed every time the REx started, the duration of the run, and what the state of charge of the HV battery pack was at the time the REx started. Based on the diagnostics data retrieved, BMW refused the warranty claim (of $13,000) because of the unauthorized code changes that allowed the REx to be started on-demand to maintain SOC. The i3 owner did not have the repair done, and apparently still drives the car with the REx unavailable.
If you do a search on the i3 Facebook group page, you will find his postings on this.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 19 inch 427 wheels, EVoInnovate EVSE