My friend has a squire strat and he wants me to put my old floyd rose in it for him....if the width of the nut is 42mm what is the string spacing?
it's not really going to matter what the nut width is.

you can put the floyd on a strat, the only difference will be about 1.5mm between the two E strings at the bridge - the squire string spacing will probably be 52.5mm whilst the floyd is 54mm. The strings will have a wider taper towards the bridge, but it should all work.

your biggest concern will be the routing... especially if you want to recess the floyd. where are you? i got a template available - PM me if you want more info