Based on the info, "Yes he is planning on building an app", I'll assume we're talking about full-stack options for an MVP web app. We could also be discussing a language for a general backend that may support web + mobile apps. I like Python for MVPs. It's an approachable language with solid, opinionated frameworks like Django and Flask you can become productive in relatively quickly.
As some have already mentioned, Go is faster than Python because Go compiles "like a bat outta hell" from what I understand. That said, it's rare to run into performance issues until you achieve significant scale. Even then, the likes of companies such as Youtube and Google demonstrate it's possible to maintain efficient production services at scale using Python.
Even though I lean towards Python, Go is a fantastic language too from what I've seen. Your friend can't go wrong either way. These conversations about which language to pick always boil down to the bias and experience of the responders. Every language has technical tradeoffs. Most of those tradeoffs can be dealt with and won't make or break the performance of the app.
It should be noted that Go was created essentially by Google to solve some performance issues they encountered with Python at scale. That said, for most applications that aren't Google scale, it comes down to the other factors, such as developer productivity.
AFAIK one of the main driving factors was the pain, they felt with C++.
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