If you are only wanting to sync music from your iTunes library, then hook up the iPhone, and on each tab except the Music one, make sure that all of the options are unchecked. All of the "Sync" options should be unchecked except for the "Sync" option on the Music tab. Then you can set up that tab to import whatever music you want to.
The only thing that you have to make sure of before you do that is that there is no Music that wa loaded onto her phone from her company. If there is, then you cannot do this. You can only sync one set of parameters from one iTunes Library. So, if you have no music from the company's iTunes library, then you can sync music from your iTunes Library. But, because the company has definitely sync'd other items from thier Library, that is why you want to make sure that all other "Sync" options are totally unchecked.
Hope this helps!
Gail - Thanks very much for your answer.
What still concerns me is that most tabs have a simple "Sync" checkbox right at the top, but the Apps tab just has something lower down that says "Automatically sync new apps". If I leave that unchecked, will the existing iPhone apps _definitely_ be left undisturbed? (It would be a big hassle for her to get them back, and she couldn't do it at all before leaving tomorrow.)