I'm a woman who has traveled to Mexico safely, including things like taking the buses and the metro.

I don't think the original poster and her friends have any intention of going to drug cartel areas. They should, of course, take reasonable care for their safety, and read up on common scams and dangers, like not taking green Volkswagen taxis in Mexico City, if they were going there. Anyway, I'd suggest you take a look at the Lonely Planet Mexico forum for more specific hints on your destinations.