The hotel zone and city of Cancun appeared, to me, to be fairly safe. Downtown was a bit scarier to my family, especially at night, but that doesn't mean it's unsafe.
I'm no alarmist, but Mexico has it's share of crime, like all countries. Away from the tourist areas can be an adventure, good or bad. I have friends who bussed around Mexico recently, and were never hassled other than watching their bus get stopped often by armed men (apparently officials) looking for illegal contraband. So, it sounds like the gov't is *trying* to inprove the safety of the country. I hope she knows Spanish well, because in the smaller towns away from tourist areas, it's harder to find English-speakers and that can be frustrating. It's too bad she doesn't go with a friend, as a single woman travelling alone could be seen as an easy mark.
Nevertheless, I'm sure you've all discussed the safety factor before planning a two-month trip, so I won't bore you with any more of my personal concerns.
Please wish her a wonderful trip and enjoyable adventures!!