Just get the source of the DLL from the “previous programmer/developer”, and open that in Visual Studio. If you don't have the source, on the other hand, it ...
You cannot get the exact code, but you can get a decompiled version of it. The most popular (and best) tool is Reflector, but there are also other .Net ...
dotPeek is a free-of-charge standalone tool based on ReSharper's bundled decompiler. It can reliably decompile any .NET assembly into equivalent C# or IL code.