Git Workflow Acquia Cloud and Acquia Dev desktop
Working on the frontend isn't tied to using Acquia Dev Desktop or
not. It's a personal preference to work locally by cloning a remote
site and work on theming/frontend and commit changes periodically to
the remote repo when you're happy with the changes you see in your browser. Some customers prefer working directly on the Acquia Cloud dev environment with Livedev. Really, it's what you think is best for you.
Git is a decentralized version control system. It means both you and your client can checkout the Acquia Cloud repo and hold a live copy of the repo. You and your client can then commit/push code either to the default branch (master) or with a feature branch. This is by no means specific to Acquia Cloud. This is how git works. What matters is you git pull always before committing / pushing code, so that you can retrieve the changes your client has made on the current working branch (e.g. master). With a feature branch, it'll require you merge it back to e.g. master when it's ready. See https://git-scm.com/book/en/v2/Git-Branching-Branches-in-a-Nutshell for details.