He's a good coach.
But lets not get carried away.
Every tom dick and harry in Scotland was saying that Murray had to improve his serve, especially 2nd serve, and be more aggressive in the points and rallies before he even got appointed as coach.
These are technical aspects to his game, but the mental side does play a part. He was genuinely overcome by the pressure in all of his slam finals prior to Lendl and had all but lost before he stepped on the court. Federer and Djokovic played well in those matches, but Murray never even turned up.
He went from that to winning the US Open against Djokovic, for his first slam, after losing a two set lead. Picking himself up for that 5th set took a level of mental strength miles beyond what he was capable of prior to Lendl.