How to work authorship of software developed with another University I am supervising a PhD student, enrolled at another University, where the co-supervisor is employed. We developed some useful software in her thesis and published 3 papers. In this development another member from the other university also participated. Now she will defend her thesis and leave, and it is not clear yet whether they will be interested in further development of this software. 

I am really interested in this and I am the only one who knows how to use it in different contexts so I can foresee that I could get 30 publications with this tool in the next 5 years. I have the feeling (and I could obviously be wrong) that since the PhD student will leave, the colleagues from the other University will not work anymore on improving the software. But if I get new publications with the software, they have the right to be included as co-authors on all the papers even if they do not anything at all. 

We are friends and we work together very well, but I think this is unfair. And I do not know how to present this situation to them. If I tell them exactly like this I feel they will get upset. So I am lost and I do not know what do here. Most optimal situation I can think is that on each new paper, tell them that I need to add some minor features to the paper (so that we work) and then I can feel they have the rights to be co authors of the paper. Are there better ways to handle this? I feel that we should have settled everything from the beginning, but it is now when I realize about this.
I don't see the problem here as one of co-authorship, but one of potential plagiarism. That is easy to avoid with proper citation. But it depends, also, on what you intend to do in the papers. 

In a similar case, suppose you found the software and the original papers online or elsewhere and you do some derived work. The originals were done by people who are strangers to you. You cite the original and acknowledge the people who form the foundation. But you don't have to invite the strangers to be co-authors. I don't necessarily see a difference here. 

If they don't participate in the new work they aren't co-authors of it. 

But, it might be worth some effort to try to convince them to participate fully and, thus, become co-authors. 

It would be a different case if you were re-writing the original papers. 

As for the software, you have to be sure that you have an appropriate license to extend it. If you haven't ceded ownership to someone else, then you have joint ownership among the three(?) of you. Get their written permission to extend it, and list everyone as co-authors (co-creators) of the software. But the software and its use are separate things. If you find novel uses of the software without the help of the others, they aren't co-authors of the resulting papers.