[Suicide Junkie]

Empire B can see whatever Empire A can see with Empire A's ships.

If Empire A has a cloaked ship without sensors, B cannot see the ship.

If Empire A has a cloaked ship, and has a sensor capable of seeing through the cloak somewhere in that system, B can see the ship.

Empire B gets to see any of Empire C's ships that Empire A would have been able to see if the alliances did not exist.