Does Sandswept Tomb work with Void Assault?

Yes, but only if the friendly unit you target with Void Assault is at Sandswept Tomb when you cast it.

Sandswept Tomb reads: "Each spell that chooses one or more units here that are friendly to it costs [A] less." Void Assault targets (chooses) a friendly unit and an enemy unit. "Friendly to it" means controlled by the spell's caster. So if your friendly unit is at Sandswept Tomb at the time of casting, Void Assault is choosing a unit there that is friendly to itself, and the [A] discount applies.

If your friendly unit is elsewhere (another battlefield, your Base, etc.), then Void Assault doesn't choose any units at Sandswept Tomb that meet both conditions, and the discount doesn't apply. The enemy unit being there doesn't help — it isn't "friendly to" your spell.