Years ago my wife got one without a problem. The only issue question they asked was how close to the US border were we planning to stay. Cancun was not a problem. That was pre 9/11 and I don't know if things have changed.
I got mexican visa also and went to Cancun, not problem if you have a Job, US$ Dollar Credit Card, and bank Acct, First time I got 1 entry visa for 6 months, and next year went again and got the reneward with 10 years mexican visa and went to Mexico City and Guanajuato.
Please do not lose the piece of paper they will give you at your entry to Mexico, the arline will ask you for that piece of paper when you are coming back home during the checkin, if you dont have it will cause some problems and maybe you can lose your flight while they find out what about your missing paper.
If she has all that and also you can give her copy of your acreditation and proof that she is your girlfriend I think all will be fine. Enjoy mexico is a great country.
