If she's a completely nooby, then the cheapest. If she's price-minded, the most expensive.
Now, seriously, it's quite difficult to recommend anything without knowing her likes. I assume you want a Xiaomi. The Redmi 3 and Mi 4 are both of 5.0", while the Redmi 3 Note is 5.5". Bigger screen is good for watching videos and websurfing, but smaller is best for the budget (even if she's going to carry it on her bag), specially if she isn't large, a smaller phone fits better in hand.
Mi 4 is still a performance smartphone, with its 3 GB RAM, its FHD screen resolution and metal case, while the entry-level Redmi 3 has an amazing battery, and more than enough power for everyday tasks.
Hope this helps.
Edit: On colors: Redmi 3 has two different 'golden' covers, Redmi 3 note has one golden cover, and I couldn't find Mi4 in other than white, but you can buy extra covers in several colors.