No. no. no way. There's no way he didn't know that Yamane had a gun. There's no way this isn't some stunt where he resurrects from the dead in three days. Besides, where did that frog guy end up? I know he wasn't present at the December 31st 2000 event, but that just makes things more suspicious! I guarantee that Friend, if he is possibly Fukubei (though I think that Fukubei is likely the face but not the actual Friend), is going to make a grandiose return.
Also, why in the world would Friend want to be on the side of the villains? I get Yamane, that guy was vicious as a kid, but a member of the original group would've wanted to be on the side of the heroes, and keep fighting against a grand cabal. Likely, someone is playing both sides.