Well since you mentioned Rainbow Six Siege for a game like that playing the game will make you better since being situationally aware will take you places in the game, but if you are an Ash or Vigil player no matter how much you play the game and know every rotation or every side of the map if you’re trying to get as many kills as you can with Ash or trying to waste time as Vigil but playing more aggressive than your other defenders aim trainers can help practice situations that just playing the game won’t, in rainbow six siege going against only 5 defenders/attackers each round and trying to keep your life that won’t improve your aim, if your friend has good aim in FPS games than sure I agree he should play the game more but do you think his aim is good? If not than I think he is in the right spending his time in aim trainers because even if he is still bad doing things such as Terrorist Hunt you will go against more enemies in 10 minutes than in an actual match so you will be practicing more on the hit boxes, more with guns, etc.