This behavior seems to come from calling git commit -v or git commit --verbose. Not sure where you're doing that in your dotfiles, but I'd recommend telling him to set an alias in his git config that does that.
If I run :set spell, I get:
spellfile#LoadFile(): There is no writable spell directory
Shall I create /Users/adamharris/.nvim/spell
Cannot find spell file for "en" in utf-8
Do you want me to try downloading it?
Select y, and it will download the file.
In which directory do you want to write the file:
Here is how I solved the problem, by adding to my .vimrc:
autocmd FileType gitcommit set nosmartindent | set formatoptions-=t
This removes the option that causes the lines to auto-wrap (which is another tweak I also made -- take out the last part including the | if you do not want to change this).
The take-away here is that nosmartindent is the trick to ...
Alternatively you can use Tim Pope's fugitive.vim. It provides :Gcommit which is a wrapper around git commit (So you can do :Gcommit --verbose).
Probably the more preferred way would be to use :Gstatus and execute cvc. Then just create your commit message and then save and quit (I use :x).
For more help with fugitive.vim I recommend Vimcasts episodes ...