PC Shuts Down When Game Starts

So here's the problem. My partner has been playing league for years, never had any problems with it, or his pc. Now he isn't even able to play a game, becouse once the match loads, his PC immidiately shuts down. Everytime. He submitted a ticked explaining this problem to support, they told him to check/update his drivers, which he did, and it didn't help. Then they told him it could be the GPU, but he monitored it and it doesn't overheat. Every other game he plays, as Overwatch, For Honor etc., doesn't cause this pc shutdown problem. Anyone having the same issue? Possibly some solutions?