Is there a gain on the sale? This might be different in each country.
If your wife is a California resident and either a U.S. citizen or permanent resident, the feds and California both levy income tax on her worldwide income, including any gain on this sale.
Mexico can also tax the gain. I doubt whether the Mexican state has income tax.
The U.S. Mexico Income Tax Treaty allows this double taxation for "nonmovable" property but the impact is offset by the U.S. foreign tax credit.
But California does not have a similar credit for out of country transactions.
Use Google to find articles about this.
Then find someone who is comfortable with foreign transactions to give you specific advice.
Ok....I need to clarify a few points here....
Due to her stepped up basis, "gain" would be a neglible issue...and what tiny amount of gain that might exist, would have most likely been wiped out by selling costs. Therefore there is really no need to address the issue of gain here.
It is more likely that the estate handled the sale of the property, and that her distribution is net of any taxes that Mexico might impose.
Those two issues however, can be addressed when she gets her consult with a local tax pro. The bigger issue is whether or not CA will impose any kind of inheritance tax.