Mexico does not have standing to sue. They filed a "friend of the court" brief, which is something very different.
Same difference to me in this instance. They have nothing at stake in an Arizona statute vs the Constitution of the USA. Nothing at all. There is no personal loss or personal liability to be lost by them.
Mexico has no standing or should have no standing in this fight, their laws have no standing. Foreign nations should never be allowed to tell us what out Constitution and statutes mean.
They are no friend of the court either. They want to impose their will on a sovereign state, Arizona. They have a parasitic relationship with the USA.