US citizens cannot play for
real money on a pokerstars even if they are outside of the US and in a jurisdiction in which online poker is legal unless they can prove residency wherever they are living. I'm not sure exact about the details since I was trying to find out this info myself a bit back, but I tniigh you would need either official permanent residency status in Mexico and/or at least be able to send pokerstars utility bills to prove you're living in Mexico (which you wouldn't be it seems in your scenario).
I wish it was easier such that if an American goes on vacation to Europe for a month they could play on pokerstars but pokerstars wants I play it super safe with the US government and therefore makes it very hard for Americans to play even legally outside the US.