A good Nasus can beat a good Lux. It just takes skill to dodge her stuff and a jungler to punish her for coming forward to smack you while you have Wither up. --- ~Victory needs no explanation; defeat allows none.~
It would be a match up that mostly came down to skill and the jungler. A Nasus with the right runes and masteries could just about withstand Lux's harass and farm under turret but that could leave him open to turret dives and being so pushed allows Lux to roam for free.
Lux could win by just having a lot of free presence in the other lanes but I don't think she can really stop Nasus farming without help. It would come down to how well they both used their advantages which is a vague and simplistic answer I admit. Lux would have the easier time in that lane I think.