I work in small team of 8 developers and we do not have direct management because of our small numbers. We are more or less self managed.
We make heavy use of version management, IDEs and all sorts of conventionally popular and well known tools and platforms. For example, we use VS Code and git for everything. This allowed us to compromise between Windows, Apple and Linux users, and almost everyone is happy.
However, we have one member who ostensibly refuses to use same tools as everyone else. He uses command line editors, cmake (manually writes scripts), perl, and some other obscure and outdated tools. He also does not follow the team coding style.
This was not problem until lately because he had separate workflow. He mostly maintained some java code and worked independently. But after a COVID related layoff, there is way more work on our shoulders and having stubborn teammate really takes toll on our productivity and efficiency. Now that we inherited pretty big C# and managed C++ code bases, having proper IDE is more important than ever. Everyone who didn't use windows dual booted it or uses our preset VM snapshot. But our teammate refuses to use windows, claiming that it is spyware. We tried to negotiate with him. One of my coworkers even offered to lend him his laptop which is set up for task, but he refused to take it. We went to upper management but they can't do anything to him because he maintains some important codebase unrelated to our team. But they can't remove him from our team either because he must fill work hour quota specified in contract.
Edit: We already missed several deadlines because of him and he's constantly lagging behind. We are unable to help him because he uses tools unfamiliar or less familiar to us. We once had a urgent need to fix back-end code he had worked on while he was taking a day off. We had about 3 hours at most, so we called him and he told us password of work laptop. It did not boot properly. After a lot of confusion, it turned out that he didn't have desktop environment or something along that lines. We found ourselves in a very ugly situation afterwards. There was some solid damage to company, our team will be disciplined, and, god forbid, we may even be sued. None of this would happen if he used the same tools as others.
How to approach this person and reasonably negotiate with him?