What is the crowd's opinion of
.NET Reflector?
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4 reviews of this app found across the web:
Ah! No, you don't get the source when you pay for Pro.
However, you can go download software like .NET Reflector (https://www.red-gate.com/products/dotnet-development/reflector/) or ILSpy (http://ilspy.net/) which allows you to read any managed DLL... like UnityEditor.dll. You can learn quite a lot from that.
Drawing boxes is documented, they usually use GUIStyle.Draw() (http://docs.unity3d.com/ScriptReference/GUIStyle.Draw.html) The tricky part is to find the style they actually use.
Would be great if you can leave feedback in VS (Help -> Send Feedback) on what we can improve in the source decompilation tools - this is an area of active investment.
Check out ilspy for an open source option which is pretty solid.
For a paid version, .NET Reflector is a solid option. I think there is a free trial.
If you're primarily doing decompilation and reverse engineering, you might check out dnSpy.
Decompiling is pretty hit-or-miss; it isn't a push-button process, it takes some understanding of the structure of a compiled executable, and specific software (which is only very rarely free). Additionally, comments are almost certainly going to be truncated, along with a number of details on variable and field names, depending on the compiler. If you're lucky, you've got a development executable; if you've got a release executable, then even more will have been truncated. It's generally likely to be easier to just get the source project from your partner.
For an actual suggestion, you might consider investing in .NET Reflector, which can decompile an exe into C# or VB.NET, but note that you'll lose lambdas, constant names, anonymous methods, iterators, async/await, and of course comments. Additionally, it's going to be a pile of source files (including for the actual built-in engine) and maybe resources, not a Unity project, and you'll be spending a hundred bucks for a basic license. That's not usually easy to navigate, unless you do this stuff for fun. I mean I do, and I still get lost sometimes.
If you just need to change something simple, you might consider looking into a free hex editor and finding out a bit about PE (exe) structures, then editing byte-by-byte. Basically find the important code and Game Genie the thing, same concept. I wouldn't exactly drop something like that on Github though!
If you want to try an open-source decompiler, look into ILSpy or dnSpy. I think that they're open source, though their performance lags behind Reflector. It might make a nice middle ground?