if you have enough screenshots to create a timeline in the chat from the moment the boss went down and rolls began right up to the moment the item was given to someone else and whatever followed is likely what they would need. timestamps on your chat would do wonders to help this.
odds are you didnt record the incident in a video but that would have likely helped loads.
keep in mind that much of the time ninja looting is not usually a bannable or reversible offense in wow as master loot is an in game mechanic to be used at the players own discretion. may not technically be fair or fun but if you join a pug with a core guild group as most of its members then youre setting youself up to be at their mercy.
I suggest to turn on Fraps with ~10-15 fps every time important boss is about to die.
Slow framerate wont affect ur gameplay at all, has low CP usage and wont flood your hard drive, meanwhile will help you A LOT in case of ninja is about to happen.
All you have to do after is to render video or sort important screenshots.
Surely it may be low framerate but good quality needed.