How to transfer songs onto iphone without using iTunes?

So my mum asked me to put songs she has on her USB on her phone. I was wondering how can I do it without putting her songs onto my iTunes because I really don't listen to them? I'm using a Mac.

Download an app called "Downloads" it has a green icon. It has an FTP server that let you upload files including songs. It even have a built. In music player.

Why would you have to listen to them?