@JLNobody,
JLN, Your wife should have no fear in Mexico City; especially in the Centro-Historico-Reforma (Zocalo) area where I stay most of the time. There are police everywhere, and I see young girls/women walking alone at night. I'm sure the penalty for crime in Mexico City prevents most crimes.
Be forewarned, however, because city maps are deceiving. What seems like just a few blocks can be miles away. Mexico City is huge with a population of over 20-million people, and probably over 20 districts.
What I find Mexico City so fascinating are their architecture, sculptures, museums, restaurants, music, bars, and of coarse the people (most are friendly). Even in hotel elevators, most say "buenos dias."
Let me know if you have any questions.