co-authorship for software adaption
As you're feeling, this is a greyish area.  To me, it depends on the paper itself.  If the paper really focuses on using this software to do something, this would nudge me toward offering authorship -- and I would ask the developer to fully participate in manuscript prep.  The credibility/knowlege of the developer may well make your paper better, and might be of help depending on how it is received by the referees.

If the use of the software amounts to a sentence in the methods section, I would acknowledge.