How to copy iTunes music from iTunes library to another, using my own music - not purchased
You can:


Send your partner the music files online (such as email)
Burn the files to a CD and transfer it to the other computer
Copy it to a thumb-drive and transfer it to the other computer


iTunes should automatically open the music files. If it does not, (I'm assuming your PC is a Windows PC) right click the file, select 'Choose default program', and then choose iTunes in the list. After opening the file in iTunes, it should automatically add it to your partner's iTunes library.