Might I suggest discussing version control with your clients in the future? "Collaboration" might be an over-charitable word for the relationship between you and the other freelancer, but even an agreement to commit in two branches to the same git repository (e.g. one hosted privately on https://github.com/ or https://bitbucket.org/) would have gone a long way towards preventing this outcome. In this case, git would have kept a permanent backup of every new feature/fix in your development process, as well as each of the other freelancer's changes. You'd have been able to merge the two at every milestone (or every day!) with minimal pain.

If you're new to version control, GitHub has a nice git tutorial at http://try.github.io/.