My friend tried to change her version of minecraft from 1.6.4 to 1.5 and now when she tries to log on it says minecraft has crashed. What can she do?

Put the summary of the bug you're having here

*What I expected to happen was... I'd go on minecraft and have fun....
Describe what you thought should happen here

*What actually happened was...it said minecraft has crashed
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ... Change back versions of minecraft
3. ...