Last night cruising in a mates commodore he chirped the rears while taking off and all of a sudden a sharp ringing/squeal came from under the bonnet, we took a look and from what i could see in the dark the AC rotating assembly thing started to tilt and as it would tilt the squeal occurred but it looked as if the belt was sliding half off.(The bottom right rotating assembly i think its the AC) What i wanna know is can this be fixed by simply tightening the nut in the centre of this or could it be something more serious that a mechanic should look at?
I'm heading to his place later to try fix it but don't know if i even can? Has anyone else had this problem? If so how was it fixed?

Cheers,
Rhys.