Well depends.
If it's a .NET codedom keylogger, it might be kinda easy to get his username/password, you can simply look at the source. If it's a .NET keylogger with a .exe stub or something that uses copying the stub and putting text to it, if it's not encrypted, you can get the username/password with bintext. But if it is encrypted, you first might want to know with what it was encrypted and then decrypt.
Most common text encryptions in keyloggers are RC4, XOR, and TripleDES (edit: free ones).