Israel needs to be careful. As friends of Israel, the United States needs to tell them that a preemptive attack against Iran will rally the rest of the Muslim world against them. Right now the majority of countries in the Middle East and around the world are against Iran getting a nuclear weapon. That is why many countries have taken steps to enforce tough sanctions against Iran. By attacking Iran preemptively, the rest of the world will shift their focus from Iran's quest of a nuclear weapon to Israel and the United States. If Israel is truly worried about their security, that focus on Israel will not be a positive thing for their security.
Great friends should be honest with each other. The United States should not go along with an attack against Iran in the name of friendship. This does not mean that the United States does not support Israel, only that the United States disagrees with a bad decision on Israel's part.