Wait, "left4dead2.exe"? Are you using Sourcemod on the client, instead of using hldsupdatetool to download the dedicated server? If so, that may explain your problem. I only run the dedicated server (srcds.exe). I've heard that running Sourcemod on the client is unstable.
If I remember right, I'm using Sourcemod 1.4.0. For the record, Sourcemod is currently at version 1.4.6, so your "1.9.0" seems wrong...did you mean metamod?
http://www.sourcemod.net/
Also for the record, SDKHooks is at 2.1. I think you need at least SDKHooks 2.0 in order to have OnTakeDamage for L4D2 (psychonic removed it in 1.3), otherwise this plugin wouldn't even work. You can find the latest SDKHooks in the link in my first post, but I'll post the link here for you again.
https://forums.alliedmods.net/showthread.php?t=106748
In case you're interested in setting up your own L4D2 dedicated server, I recommend crash2004's guide.
http://forums.steampowered.com/forum....php?t=1381634
EDIT:
I've also been wondering...the original intention of this plugin was to preserve friendly fire in general while taking care of specific situations where a server owner may think that friendly fire shouldn't happen. If you're setting the friendly fire factor to 0, this plugin isn't of very much use, then. With the default settings, the only thing the plugin would do is blocking 0 damage effects (no vocalizations from target, no record of friendly fire in the stats, no recoil when survivor bots shoot through you).