It seems like Python works for him, so he should continue using it. I'm not a purist who thinks that absolutely everyone should use one language (although I dislike Java).

There's nothing (major) wrong with Python. It has some quirks, and I don't like the lack of tail recursion, but it also has a large community and mature frameworks. If you need to build something quick and will rely heavily on existing libraries, it's a great choice. If your project will have to scale and is guaranteed to go multi-developer, then you probably want Haskell's static typing.