Having your rails app provide a RESTful API is a good idea here. Your standalone front-end app can then interact with the API over HTTP(S).
If you want the front-end app within the rails app but need repository separation (i.e. don't want the front-end developer to access the code of the rails app), using a git submodule may work but probably needs some organisational thought.
This is what I'd do:
First clone your rails app from GitHub or Bitbucket (or git init one locally) and then configure a git submodule.
git clone git@github.com:pathto/myawesomerailsapp.git
cd myawesomerailsapp
git submodule add git@github.com:pathto/mystandalonejsapp.git app/assets/standalone
Now when you
cat .gitmodules
you'll notice there's a new submodule configured in your repo.
Commit and push your changes. Ninefold will detect the submodules and use them, but if you have any problems just get in touch.
Good luck!