If 4G don't mine, then both are equally ok.

Here are the verdict:

Consider Zenfone 2 over Mi4
1. Bigger screen, 5.5" vs 5"
2. Has micro SD slot
3. 32GB storage vs only 16GB storage (more storage memory is always welcome, but Mi4 only come with 16GB with no expandable option)
4. Has NFC (may not be a drastic feature at the moment, but future proof, Mi4 don't has this)
5. Has 4G (I know your friend don't mind, but 4G is the future, and it is proven to be much faster than 3G in overall internet usage)
Consider Mi4 over Zenfone 2
1. Much lighter, thinner and slimmer, easy for 1 hand use, much slimmer screen bezel
2. Better camera quality
3. Around 50% more brighter screen (well, Zenfone 2 screen is not that dim actually, but much dimmer than Mi4)
4. Better build quality (has side metal frame)

Overall, Zenfone 2 win over Mi4 in features, but Mi4 win over Zenfone 2 on better quality on build, outlook, and camera.