When is it ethically acceptable to fork someone else github code and continue with that?
It is always ethically acceptable to fork any code on Github (or Sourceforge or wherever you got it from) and do whatever you want with it, within the terms allowed by its license! This is what open source and permissive licenses are all about. The original author wanted the world to contribute to this piece of software, and you will be doing exactly that.

Fork it, respect the license requirements, and go on with it. It does not matter whether it is abandoned (like your example, since the last change was 3 years ago) or current.

If you do feel that you did some good improvements, send pull requests back to your upstream, as  usual. If the original author never comes back, then so be it. If he does, he can integrate them (or not if he so chooses). But this is strictly optional, you are neither legally nor ethically forced to do so.

About uploading it to some other locations (Ansible Galaxy or whatever - say Docker Hub for docker images and so on): the issue is probably mostly to avoid irritations due to the name being "replaced" (but I'd assume if the original author already has uploaded it to A.G., you can't just replace it, anyway).  So, in the latter case, just upload it with another name, which is still easily identifyable as related to the original - say, append some suffix which is not too obnoxious.

If the author, in your case, should later re-appear, and you had uploaded the software to the Ansible Galaxy, and he wants to take over, then you two will simply communicate and resolve the issue somehow (i.e., merge the two forks again and decide who keeps updating Ansible Galaxy). Chances are that he will be more than glad to leave the work to you.

Oh, and to be sure; this piece of software is under the MIT license:


  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:


There you go.