That's exactly what I am thinking. He must have seen everything since he was walking behind them, as per the ladies story, as they left the Be-Bop bar with the victim. Fuen must have seen everything, but seems to have dissappeared from the case and not mentioned except by the wounded lady.
Surly the policemen must have seen Fuen, and if it was the policman's intent to eliminate witnesses by shooting the lady, why was Fuen not shot as well?
But let's not rush to judgment.
According to one of the latest articles, Fuen was at Reisig's bedside in the hospital and was quoted in the interview.
To take the opposite angle - for all we know, he might have been too far behind to do something, or took cover when the brawl started, or when the first shot was fired, or might have run away to get help from friends.
I still come to the same conclusion as you, let's not rush to judgement, we'll just have to see if more info comes to light.