i know what you mean, when youre having a good fight and some monkey blows through from above and steals it right as you are about to have a shot. then i think its fair to have a little pop as you go by just to see who you were fighting, if you hadnt hit him already at some point.
if you get the kill then the uninvited picker has no leg to stand on anyhow.
now if the other friendly is helping you when you are losing, making the bandit turn off you so you can come around safe instead of running, or such like performances, then you dont have a leg to stand on stealing his kill when you would have died 1 on 1.
geuss its part of the ROE here that some people find easy and dont even think about and a few randoms totaly ignore.
at the end of the day is combat with planes, there isnt a rule book.
act in whatever way you like as long as you feel good about it. if you steal one of my kills i would call you names on my squad vox but let you have your fun unmolested. i can find plently more. If you try to steal it and miss i will taunt you mercilessly in public

S!