well i think that's weighted on if they are a friend. the word ally is bandied about so much that at the end of the acronym LGBTHGKJSHДФEDA it means "and allies" but in terms of international relations, it has more specific meanings.
allies have to be part of an alliance, be it full fledged will stand by you even if you go get into a drunken bar fight and it was totally your fault, or merely a defensive one. the US and mexico have no such agreement. all that can be said is that they are a neighboring country. with NAFTA certainly strengthens bonds of friendship, reneging on deals, like TIAR, breaks them. canada, our friend and ally, is not part of TIAR. it is their prerogative. they are, however, part of NATO, of which the US is the only dual member of NATO and TIAR. this has to do with zones of influence, and the fact the US is well... the US. with canada's northern sea-icy border with Russia and historical links to europe, NATO membership made a lot of sense. with america's historical place as hegemon of the americas, it made sense for the US to be in TIAR. Now, can TIAR be better? absolutely. but i'd be wary of mexico in the new TIAR. mexico has largest army in central america, and the 3rd largest army on the continent. that they didn't join when their neighbor, their friend, was attacked, is puzzling