Plenty of HFT shops do not require C++ from the quants who are only involved in modeling. As you hypothesize, Python is sufficient for researching models. Your friend may simply be at a place that values tighter integration between research and engineering. Other shops will just have a Python wrapper to fetch historical data, obviating the need for C++.
I was able to reach out to a friend working at a HFT place as a quant researcher. It appears that C++ knowledge is key to interact with the infrastructure (which is all written in C++). For example, when trying to conduct a research on Level 2 data, researchers are expected to know C++ to be able to use the C++ APIs to aggregate the relevant order book data ...
C/C++ are low level languages in comparison with Python. Moreover, Python is scripting language. By nature, scripting languages are slow becuase they have to be interpreted. As there is a requirement for the highest possible performance in HFT, compiled languages are natural choice.
Concerning libraries, there is also vast amount of maths libraries for C/C++...
Let me give you the perfect solution.
The charting, graphing and analysis can be done using the PyLab environment.
You can integrate the code into R using the package called rPython.
You can integrate it to C and many other languages.
Python also comes with infinite more features. So instead of looking for a particular library, use Python.