#1
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?
#2
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