I'm currently working on a project with a team that's using a git workflow. It's fairly simple, master should be in a deployable state and branches are used to create features and hotfixes. Whenever we have a feature or bugfix completed and tested then we move that over to master as soon as we can. The idea is that branches should be as small as possible to make it easier to merge them back into master. We have a policy that any code pushed to master branch should be in a deployable state and pass the tests.
We have got a situation where one of the developers has done a lot of work (a few months worth) on a single branch and this branch hasn't been merged back into the master yet. There's now a few separate features and a bunch of commits on this branch, essentially this branch really should have been merged back in a few times already but so far hasn't been. Most of the code is in a good state with unit tests that could be merged back into master but the most recent changes should certainly not be as they are not completed and aren't tested.
What is the best way to deal with such a situation where one branch is really far away from the others? What ways can we avoid branches getting a very large number of commits away from master in the future?