No he need to maintain only the 2010 version and when installed on 2016, the references will be updated. As consequently the add-in will not be opened on lower version this will not be a problem.
I speak from own experience - I maintain addin only on 2010, and then it is updated without problem on PCs with 2013 and 2016.
Added benefit is that with addin you can implement auto-update functionality. see http://www.jkp-ads.com/Articles/UpdateAnAddin.asp
Bookmarks