You have just proven his argument for him.
No, not really. I put that in there as a trap of sorts to see if he'd read any of the rest.
Of course the answer I expect (which appears to be the one you expect as well) is that he feels he would have the "right of way" in a situation like that so would therefore be "wronged" if someone came in and involved him in a collision (making the person diving in responsible or "at fault").
That's just silliness, of course. If it were a RL situation an attitude like that would get him killed! In a situation like that he would of course need to protect himself first and foremost, even if he was already engaged.
First of all, he's fighting someone who's potentially "being rescued" by the person he's about to be involved in the collision with. The guy coming in should be expected, and should be watched for. Shooting the guy in front of you is a poor excuse for getting shot by the guy behind you...
Second, diving in faster is going to make the other guy less maneuverable, just as being extra slow would make someone less maneuverable. It should be expected that someone diving in with excess speed will not be able to divert much from their path (at least not quickly or suddenly) if they're fast (and if they're not fast they have enough control, forethought, and likely skill, to be a low collision risk anyway). That's a predictable situation, and it shouldn't shock or surprise anyone.
Third, it's a basic, known requirement that to be successful you need to avoid collisions with enemy aircraft, and the landscape. It doesn't matter one bit what type of aircraft it is, it's speed, altitude, heading, or intent. It's YOUR responsibility to act to preserve YOURSELF;
it's not your opponents responsibility to make sure that you don't get damaged.
Fourth, regardless of the situation lag will be a factor, so your opponent won't be exactly where he appears, and he won't see things exactly as you do. That doesn't change just because you're flying a spit, or because you're low and turning while he's fast and diving in. He may fly past you collision-free on his end (having successfully avoided colliding with HIS opponents) while you take damage because you weren't able to successfully avoid colliding with YOURS. It's not HIS fault you couldn't or wouldn't protect yourself (or that you never recognized the threat).
Fifth, whether it looks (on your end) like you collided with the other guy, or whether it looks like he collided with you, there's almost no chance in the world that the situation and timing looked identical on his end. Even if it looks like the other guy dove down and flew right through you, it probably didn't look like that on his end... Even if both pilots see the same collision, it's unlikely that they'll see it identically. You saw me hit your tail, I saw you hit my belly... Neither is at fault, but both are at fault? Both cases may be true, but neither is possible if the other is true.
Sixth, identifying "fault" is a very vague concept... What if the pilot diving in flies a very safe path (no threat of a collision, although he'll pass very near his opponent), but with enough speed that he's pretty locked into maintaining course and zooming on through. Then, at the last second the lower/slower guy changes heading and diverts right into the path of the guy diving in. The guy diving in hits the slower plane, but only because the lower guy diverted his path and placed himself in danger (not necessarily on purpose either; maybe a stall dropped a wing suddenly, etc). Who's at fault? Not the fast guy with the stable, predictable flightpath, but also not necessarily the guy who diverted into his path.
When it comes to collisions between opponents, the only real way to lay blame is to have a "right of way" rule that says that one player has a higher "right" to a location in space (and time) than the other. It would need to be a set of "rules" very similar to traffic rules where everyone is trained to know who has the right of way and who is required to yield... And of course that wouldn't work in a situation where we're shooting at each other... Would it be the high guy that must yield to the low guy? Or the slow guy? Or the guy on the right? Or the guy in the xyz plane? Or the guy pointing closest to due north? Must the more maneuverable guy yield to the less maneuverable?
If nobody has the right of way, then nobody is required to yield. So who's at fault?
In the end, you really cannot assign fault. If I dive past you (but miss you cleanly on my end) how can it be my "fault" if you see a collision and die on your end? I flew fine and succeeded in avoiding a collision (even if I was fast and you were slow); you didn't.
