There are a lot of reasons for Pakistan not to use nukes. Including a neighbor with a lot of nukes. India may not care about Iran, but they may find Pakistan using nukes at all and irresistible provocation.
If Pakistan nuked Iran the whole World would shake its finger at them while being privately pleased another potential player is out of the game.
India would not use nukes, but it allies with Iran as
1. long civilizational contact
2. the enemy on the other side of Pakistan.
For #2, this is really good for India