I’m going to go out on a limb and suggest that the item doesn’t exist. I have never heard of it; while items are certainly not my forte, I know a lot of 3.5. Furthermore, I can find nothing with a Google search: no forum discussions mentioning it, no play-by-post games using it, no illegal compendiums giving its stats when they’re not supposed to. Nothing. I could be wrong, but I will believe that when and if he produces a citation for the item.
On the other hand, there are definitely rules for garrote wire. That’s in Song and Silence; technically it’s a 3.0 book, but the garrote hasn’t been updated so that makes it legal for 3.5. It is basically a weapon that allows you to grapple and deal 1d8+Str damage with the grapple. Not really all that impressive; certainly not capable of decapitating someone.
Also, doppelgreener makes a good point in the comments: it’s pretty common in RPGs to rename or reflavor things. He may be doing that (but should have told you if that’s what it was).
The thing with the harness and using it like a noose sounds like a situational advantage he was trying to give himself: don’t let that fly. I think it’s good for a game if the players can generally assume things work by the rules (emphasis on generally though), but I doubt there is any rules for the specific stunt that he pulled. Depending on he set it up, maybe the DM might give him a circumstantial bonus or something – that’s the kind of thing that makes RPGs cool and why DMs are better than computers. But it must be the DM’s call – players don’t dictate circumstances like that (in Dungeons & Dragons, anyway; other systems allow that in specific ways, usually involving some form of resource cost to allow them to modify the environment).