How can we download that music to her iPhone?

In iTunes on the Menu bar --- File > Add to Library... > Select the songs.

Once added in your iTunes Library

Connect your iPhone via Apple USB cable

Click on the iPhone icon

Click on Music

Sync Music

That's it.

Sync or remove content using iTunes

1. Open iTunes and connect your device to your computer with a USB cable.
2. Click the device icon in the upper-left corner of the iTunes window. Learn what to do if you don't see the icon. 
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5. From the list under Settings on the left side of the iTunes window, click the type of content that you want to sync or remove.1 To turn syncing on for a content type, select the checkbox next to Sync.
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9. Select the checkbox next to each item that you want to sync.
10. Click the Apply button in the lower-right corner of the screen. If syncing doesn't start automatically, click the Sync button.

1. You can only sync your iOS device with one iTunes library at a time. If you see a message that your iOS device is synced with another iTunes library, your iOS device was previously connected to another computer. If you click "Erase and Sync" in that message, all content of the selected type on your iOS device will be erased and replaced with content from this computer.

Sync your iPhone, iPad, or iPod using iTunes on your ... - Apple Support

https://support.apple.com/en-in/HT201253