It is absolutely not immature to do that. Your colleague is obviously understanding the code he wrote for now but even he will struggle with it after putting it to rest for a week or two. Properly indenting and grouping code is extremely helpful for understanding it. I say that as a programmer with over a decade of experience.

You might want to read yetanotherchris.dev/clean-code/ges... and maybe talk to your colleague about it.