You have to do more than just have the cops called to your house to have it hinder a firearms purchase.
I'm guessing there is more to the story.
With several others there as witnesses, nothing happened other than questioning, etc. and then everyone was let go. The guy has a 100% clean record, there is nothing other than this incident that could cause it. So, it's either two things. First, he somehow got tagged during this encounter with police. Or second, there is an error in the system tied to his name. When I sold firearms, I had a few guys who would get denied and then follow the appeals process. Most often it was just an error somewhere, name issue, etc. so it can happen. My friend has never tried to get it cleared up but has been "told" by some that the incident referenced probably is causing his issue.
As I said, I'm all for restrictions for valid reasons. Just make sure you can sort out the cause and go through the process if you have an issue.