Why not learn R? In the last year I've spent around 80% of my time working with R, coming from the last five years almost exclusively with Python, and there are some great reasons to use R. Although if the tidyverse didn't exist I'm not sure I'd be saying that. I find that suite of packages together to be a very cohesive set of tools for doing data science.
Three reasons (I know both languages well): (1) R is used much less in the data science industry, and (2) Python is a more universally useful language. If he learns it for data science then he can easily write utility scripts, build a back-end, etc. (3) the overlap between R and Python capabilities is so significant it would be a waste to start with R, I would only suggest picking it up if he needs some niche package that he can't get in python.
