You need to convert it first to the iPhone 6 plus format. Second, you need to copy the video file ( converted) to iTunes. Last , you need to copy the video file to your 6 plus via movies tab in iTunes by selecting the check box next to the file. Hit apply. Let it copy and once done. Eject it. Your video file should be on your phone by launching the video player.
Sent from my iPhone 6 Plus using Tapatalk
How is he going to install iTunes on a Chromebook?
I don't think any of this is correct.
From your Chromebook, go to icloud.com. Go to photos, and upload the photo from your video location to your iCloud Photo library. Assuming iCloud photos is enabled on your iPhone, the video should appear in your library.
If iCloud.com upload doesn't work from the chromebook (never tested it). Upload it to whatever cloud storage service you can get to via web browser (dropbox, google drive, onedrive, etc), and install the same cloud storage app on your iPhone, and save the video to your iPhone library via the cloud storage app. Shouldn't have to convert it.
This all assumed you have a wifi connection to sync your cloud storage up and down on all devices.