OK, you would not necessarily need to root her phone, but as for her Google account password, it's easy to retrieve it / reset it - go to google.com, use the link at the top right to sign in, then use the forgot password to initiate the reset.
At this point, unless she really doesn't mind you rooting her phone, I'd opt for a Factory reset - but that will also require her to use her Google account password. I'm betting that is it not cache so much as it is data that killing her phone - things like call logs, SMS, and data for apps.
If you go into settings --> Manage applications and check each individual app out, you can see how much space is being used for data - that will tell you who the big offender is.