How can I avoid "Windows protected your PC" problem when my friends try to use my executables?

This isn't an error; this is a design choice by Microsoft to protect those of us who are not tech-savvy (i.e. potentially your friends). And although it is a little bit annoying, it is not actually a roadblock.

When you click more info,

you will be met with

and thus be able to run the program by clicking "Run anyway", despite Microsoft's warning, because you know for a fact that the program is safe - or in this case, your friends know that the program is safe, because you probably don't have any malicious intent.

... Or do you?

Its due to windows built-in security feature. Try not to send .exe but send your code and your friend could compile it at their own. Windows just not authenticating it, as the source is not verified, according to your friend OS.

You need to disable the windows smart screen to run unpublished apps in Windows 10. Follow the below steps for Windows 8 or 10. https://www.howtogeek.com/75356/how-to-turn-off-or-disable-the-smartscreen-filter-in-windows-8/

Or you can surf the internet for disabling the windows smartscreen with yours or your friends OS build number.