I think some of you are overthinking the friendly fire thing.
Once, when under fire, the PD came up from behind. My pistol was in hand and I was behind cover. I saw them and waved them in. It was obvious to them that I wasn't the one firing the shots. Point man came up, and as I holstered. pointed the area of interest out, and gave a sit-rep, he waved the others in. I was thanked and went to the rear for debriefing.
It's not rocket science, unless you're charging down the mall yourself.
The reality will be one of 3-4 things.
1. You'll be in earshot of the event and evacuate.
2. You'll be downrange at a distance and take cover and shield whoever you can and I assume take up a position from cover. And unless your position is untenable due to an advancing adversary, you probably will never fire a shot.
3.You'll be downrange and it'll be you and him, along with all the other poor souls near you.
4. Or, just maybe, you'll be in his 'four-to-eight oclock' and if offered a clear downrange, you can engage. Maybe.
There will be many variations, but for us civilians to fully engage like Nanuk will may lead to complications, so to speak. Like charging down the mall and catching friendly fire. Unless you're with, not near, but with a LEO and helping him. Keep word is 'with'.
A for instance. Wife and I came up on a B&E. Told PD was several minutes away. I told dispatch we'd stay in the vicinity and try to get any descriptions. We parked 1/2 block or so away where we could see the front, one side of building, and some of the rear 'yard'.
First responder arrived and backup said he was 10 minutes away. (I have a scanner.) I flashed my lights, stepped out and slowly drew and held my weapon up, flat-side out and said I would cover him from the sidewalk. He reported that and went in with me covering the doorway.
When 3-4 more cars rolled up, I went to a relaxed position, weapon pointed up and most went past me into the building. At least two thanked me as I reholstered. I stayed outside and gave a report when all was 10-4.