Honor Level with Friends?

You claimed that a report is valid even if it does not pertain to an actual offense

No. Don't put words in my mouth and then say I'm assuming things. Let's double all the way back to the start of things here.

...Even if he gets honored every game, if he's still getting validly reported* as well then that will slow down his honor progress.

*For clarity I mean if he's still getting validly reported for behavior that's poor but not poor enough to warrant a punishment.

Prandine mentions that negative behavior can validate a report without directly causing a player to be punished for it, and that these valid, but not punishing reports can hamper Honor progress.

A report for behavior that's poor, but not poor enough to warrant punishment isn't really a valid report.

You then try to argue that these lighter offenses should not be considered valid reports.

No, it's still a valid report. Riot expects a certain degree of composure and sportsmanship from their players, and falling below that line means that you're not being honorable - hence, being reported for falling below that line, however slightly, is enough to slow Honor progress.

Then I explained that it's still a valid report, as simply not being punished for it doesn't mean it's good behavior, and that the IFS does slow Honor progress because of that.

I tried to explain to you that being negative and below the line of expected conduct is a punishable offense - the only reason you could possibly construe that I said "a report is valid even if it doesn't pertain to an actuall offense" is that you're pushing subjective ideas into this equation.

You may not think it's punishable, that it shouldn't constitute a valid report, but the fact of the matter is that if you are below that line, if you are being negative, even if you aren't getting punished, you are breaking the rules.

The IFS sees these reports, and if they aren't directly punishable, they still result in decreased Honor gain, because, again: _If you are below the line of Riot's expectations of conduct, you are not being honorable._

...as you ignorantly assumed.

And since it's clear that you'd rather insult me and put words in my mouth, I'm calling it here.

End of discussion.

There is no reasonable excuse for it also to slow honor progress.

Not being honourable is absolutely a reason to slow honour progress. It's not rocket science.