Do not give out advice related to cardiology because in this case you are wrong on both counts.
It's pretty reckless to tell someone that a-fib is immediately fatal. A-fib is NEVER immediately fatal. Some people live in a-fib 24/7 for years. It's not pleasant, but do-able when all remedies are exhausted.
And it could be a form of tachycardia, which is just another way of saying HR is fast.
Electrical interference that I've seen results in a brief spike in the HR display, to the highest number the HRM will display, around 229 for my Polar HRM. Seeing the HR come back down in steps is not something I've seen from interference but I have seen it as part of my previous symptoms for a-fib and atrial flutter.
Typical for a-fib episodes is that you would feel a fluttering feeling in your chest just prior or during, but maybe not. You can check your wrist or neck pulse during the episode. A-fib will feel like irregular beats, irregular or random spacing between pulses.