Update: this situation is solved

Read how it all ended at the bottom of the post.

Background

I am a graduating Software Engineering student, working as a backend application developer. About a year back I encountered this volunteer organisation by veterans for veterans. I cringed at the state of their IT. In fact, there was no IT at all, yet this organisation is based around their Excel-based administration. I offered to help them with their IT issues, which they gratefully accepted. Over time, I managed to help them with various issues, like having them work in a shared document instead of mailing the Excel document over multiple times a day, removing viruses from their computers, but their main issue remained: their Excel-based administration system is bloated, difficult to use and full of mistakes. I offered to have a look at the possibility of rewriting it as a web-app, explaining I am a student and a full-time worker, so I wouldn't be able to spend more than a few hours a week on it and that I have very little experience with web-apps. I told them it could potentially take a couple years. An IT guy with Excel experience also joined and made the Excel a lot better to use, while I continued on the long-term web-app.

Over the past year I have spend quite a bit of time learning PHP, REST APIs and various other necessary skills to build the web-app. It is not going fast, but it is going well. I have completely set up the host and I have optimised various DevOp things like automatically running unit tests and uploading to the server on a GitHub push.

The chairman now wants a website with information as well. He has recruited a good friend of his, let's call him Steve, who, the chairman says, is 'good with HTML', who will work alongside me to make the website and the web-app work together.

The issue

The chairman has given Steve access to the host, even though he would not need that, as I have set up 'uploading' to work through GitHub. Apparently Steve has cancelled the host and pointed the domain name to his server running WordPress. All my DevOps work on that server is gone. He seems unaware of what GitHub is, he completely ignores existing requirements and does not communicate with me at all.

The API for the web-app is pretty far done, but, as with any API, it does not have a visual part. Meanwhile, the chairman is over the moon with Steve's 1-page WordPress website. He thinks Steve is the best software developer ever and I can learn a lot from him, but Steve is actually a WordPress website-maker. The Excel-guy is aware of the extend of his knowledge, but it seems Steve is not. He thinks he knows more about web development than me, because he has made multiple websites for actual clients (and he is good at that), but he does not know for example what a HTTP request is or how version control works. (I am quite certain he knows a lot more about WordPress than me though!)

What can I do?

I would like to convince the chairman and/or the rest of the volunteers that Steve is heavily distrupting and deleting my work. Unfortunately, besides the Excel guy and Steve, none of the volunteers are tech-savvy at all. Most of the other volunteers believe Steve is a far better developer than me. Perhaps the age difference (twenties compared to sixties) has to do with that as well. I have very little credibility left as a 'website builder' (they don't know the difference between WordPress and a web-app) with the volunteers, because of Steve's downtalking and because they cannot 'see' my API. I have suggested a Skype meeting before (since Steve lives far away from the other volunteers), but Steve 'does not have Skype', so this is not possible.

I don't want to abandon all the veterans who need my (indirect) help, but I also want to leave this constant source of stress behind. How can I regain credibility and convince them Steve is actively disrupting my work?

Update: how it all ended

I know you're all interested in how this ended ;)

I spoke with Excel-guy on how to proceed. Excel-guy didn't really follow what happened, but understood I wasn't happy with Steve's actions. We would call about it later. It only took a little bit of time for more problems to surface: Steve's actions also made the email unusable. With the chair and Excel-guy getting swamped with phone calls by volunteers, they quickly figured Steve accidentally created a small disaster.

Meanwhile, a new web developer has joined the team, another friend of the chair. He seems aware of what he knows and doesn't know. He admits to not knowing PHP as well as HTML and not knowing GitHub, but he believes we can learn from each other and I agree. We've decided that a web-app may not be the best way to go. Instead (thanks to the tips of some amazing people here), we've decided to move forward with a PHP-driven Google Spreadsheet.

We've concluded I have most experience with software development in general, Excel guy with Excel and Steve and the new guy with websites. I am in charge of the back-end, Excel-guy is going to be in charge of the Google Spreadsheets-based frontend and we will closely collaborate.

Steve has been put in charge of the website together with the new guy. Everyone has been explicitly told not to change any configurations without consulting all the other IT-people. All those decisions must be approved by everyone now. We're going to work with sprint-like deliverables, so non-technical people can see progress too.

All in all, this has been a huge learning opportunity for me.