An unspotted enemy sniper is firing on an adjacent card, a friendly squad from a different card moves onto the snipers card, what happens next?
1. Because the enemy sniper is unspotted, it follows 9.6 and continues to exert it's external PDF on the original targeted friendly unit.
2. It follows 6.2.3 p.32 "A firing unit loses its PDF if an enemy unit enters its card. It automatically shifts its fire to the unit that entered its card"
6.2.3 is overruled by the Sniper rules 9.6
A sniper remains in play and continues to attack the card that brought the sniper on the map until spotted.
If no targets exist on the target card the sniper will engage the closest friendly occupied card in its LOS. If multiple potential targets exist, engage the one with the most Steps. If there are still multiple targets possible, determine the new target randomly.
You need to spot it. When you do it will try to get out of dodge, tsnipers are not governed by the Enemy Activity Hierarchy.