Broken merge commit

So I'm not sure how your developer managed to add the non-merge; it is as if he passed --squash to git merge, though that is pretty hard to do by accident.

Anyway, normally I would try to revert the merge, but what you want to do is revert the "merge" relative to the second parent, and there is no second parent in this non-merge. Reverting the real merge would have no effect, since it did not change any files. So lets try doing a "hand revert" of what the merge would have been. Let's say the fake merge's hash is abc, and the real (but empty) merge's hash is xyz. If we run the following:

git diff -3 abc xyz^2 | git apply 

Then git will (hopefully) undo some of the damage. What this command does is take the steps necessary to bring the state of the fake merge to the state of develop before the fake merge and apply those steps to the working directory. If it applies cleanly, you can then add/commit the changes. The -3 option tells apply to attempt a 3 way merge if the patch does not apply cleanly; you may want to try it both with and without that option.

Of course, it may be the case that there have been so many changes/merges since then that this doesn't help. But it's worth a shot!

The selected commit message says merge but doesn't contain any reference to a commit that is merged

This is a special commit named "merge commit"

Read all about it here: https://git-scm.com/book/en/v2/Git-Branching-Basic-Branching-and-Merging

Instead of just moving the branch pointer forward, Git creates a new snapshot that results from this three-way merge and automatically creates a new commit that points to it. This is referred to as a merge commit, and is special in that it has more than one parent.

The merge commit is created when you choose to work in a no-ff (no fast forward) configuration.

The --no-ff flag prevents git merge from executing a fast-forward if it detects that your current HEAD is an ancestor of the commit you're trying to merge.

A fast-forward is when, instead of constructing a merge commit, git just moves your branch pointer to point at the incoming commit. This commonly occurs when doing a git pull without any local changes.

To understand the differences between ff and no-ff take a look on this: