Conceptually he's right, you want to go after the weaker partner in an alliance and find ways to pry them away from the other instead of bashing your head against stronger one. However, he doesn't quite have a grasp of geopolitical realities. Ending the war in the Ukraine in a way that's both acceptable to Russian AND weakens the China-Russia alliance is not gonna happen, you can have one or the other, getting both is about as likely as your wife letting you have a harem. At best, he can end the war without affecting the strength of the alliance either way, and in time there should be other opportunities & ways to reach his goal.