Two laptops authorized to play my iTunes library; what can each laptop see?
You've actually posed two questions.

For the iTunes library, I'm assuming you've gone to the menu and Authorized This Machine to access your content. This authorizes the computer to access your paid content, meaning movies, tv shows, and songs downloaded through iTuens or via iTunes Match (if you enabled that on this machine too).

For contacts from your phone, did you enable iCloud on your old computer? If so you could have possibly enabled cloud syncing of contacts, mail, and calendar events. If you did this, then your friend will be getting whatever information you enabled. Thus to answer your other question, in this case she will automatically get your information.

Keep in mind that the Home Sharing feature, iTunes Match, and iTunes authorization End User Agreement License covers your household; thus, by allowing your friend to access your content you're technically violating the terms of service agreement.

If you want to to maintain your privacy, on your old machine go to System Preferences > iCloud and sign out. This will take care of your privacy concerns of contacts. In iTunes, go to Store > De Authorize This Computer, then Store > Sign Out