So, we are an NGO and we develop the new IT system for it on a voluntary basis that our members will use. It is a must for us, as people are not satisfied at all with the old system. Nobody has any salary from developing, it's all voluntary.
At one point I decided to rewrite one of the parts of the system that was used before. My point was, this part of the old system was not integrated with the new one, and I can re-write it to remove everything that is not required and to implement only these features that we really need and will use. I also know how the people would use the system, as I worked with it in almost all ways the people would use it.
I've got some feedback from the people who will use it and started implementing it. However I made a big mistake and did not ask for any feedback from the developer and the maintainer of the old system (it is one person) about that. In fact I didn't even let him know that I'm working on rewriting it. That was a result of miscommunication, as I didn't know what he was working on, and vice versa.
After I've implemented most of it, the headquarters of our NGO approached us and asked if we have something working so we can use the new system (specifically, the part I've rewritten). I said yes, because I thought it was mature enough.
Then we announced to all members that we will use the new system. And then this old maintainer answered our letter, asking to clarify if we are using his system or some new one.
I realized at this moment that not communicating with him before was a big mistake and decided to write him a personal letter to apologize and ask him to join the new development, if he wants to. As a response, I got the email full of personal insults, where he claims that I'm not capable of collaboration, that I should've gathered feedback from anyone, that rewriting the system is not as easy as it seems and that I'm not smart enough to do it, and that he'll stop working on it.
Now he is hostile to our team of developers because of what I did and he answers in the passive-aggressive way to everybody, blaming us.
Moreover, now some people are asking us to resolve this relationship problem, as he does a lot for us and in one of letters to us he told that our project management is a failure and he won't do anything for us anymore. And I cannot do it, because if I'll mail him, I'll get a lot of personal insults as a response again.
To clarify: the part of the system I'm working on isn't bad and it's not like I don't have enough knowledge to build it. It's working quite stable and these issues that users of the system have are resolved quite fast (I'm doing everything I can to make it as smooth as possible).
I'm not sure if I cannot do anything about. What can I do in this situation? I've apologized already, but that didn't work.