I would advice to leave this decision to the bitter end, first layout the architecture (monolith, services or serverless..), choose technologies and then the languages.
For example if he does the front end too he will have to learn JS too, so he could minimize the time to product by choosing JS (nodeJS) instead of Go or Python just because he doesn't need to learn another language.
There are also different styles of programming, communities sizes, tools and environments to be considered on a specific level (his personality, knowledge, preferences and the app requirements) for Go vs Python.
But most likely he doesn't want to hear and consider all that, so I'll comply and just respond Go.
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