[Gameplay] Nami ult shows that it is off cooldown in one client and not another

I used a google docs to insert the images since I couldn't insert images here.

If you notice, there are two gameplay clients up: mine (Chris the human [Amumu]) and my brother's (Charleszard [Nami])

At 31:46, I am screaming at him to ult. He is yelling that he doesn't have it. He says he ults the second he has it.

After the game, we go back and look. My screen says he has ult. His does not. His ult comes off cooldown at 32:01.

Here is a plays.tv link:

http://plays.tv/video/57fc4ac0287dc629c9/-bug-nami-amumu-ult-client?from=user