So basically I needed to take my mates guitar apart cos there was a string locked in the whammy. He put it back together again and forgot to include the springs. He hasn't noticed any difference in playing but is it bad if her doesn't put them back in them? and how do you put them back in?

Its a cheap custom made single cut with a whammy if that helps, cheers
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loosen the spring claw in the trem cavity, hook the springs on the trem block, then the claw, and tighten the claw. All you should need is a screw driver.
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^That's true, but I'm wondering- how can there not be a noticeable difference?
Some cheap Strat copies don't have trem posts for the bridge: instead they have six screws, one under each string, like this http://home.roadrunner.com/~acote8/OldBridge.jpg

If these are all done up very tightly, it'll stop the bridge from rotating at all - though intuitavely I'd say you'd probably need a light string gauge for this to work.

If that's not it, I don't know. If this Is what's happening though, Do put the springs back in as otherwise you're putting far too much turning force on those screws.

As for putting them back in, as Robbgnarly said: put the point of the spring in the hole in the trem block, then pull the loop over the spring claw. Remember to evenly space the springs and make sure the trem claw is level!