They don't make rules for everything because they want the GM to make stuff on their own, too. This is one case where you're gonna have to throw something together. It wouldn't work like a garrote at all based on what those are, and I totally understand what this player is trying to do. I know pathfinder has suffocation rules but I don't think they have any on strangling for PCs with weapons. While you could look at things like the garrote and the choker monster, this is a chain with spikes on it. It would be tearing flesh at the same time as choking. Also due to the only two safe holding spots on the chain, it would be hard to use as a strangling item I think. Unless the person is laying face down, spike chain across the throat, and the player's foot on their back while pulling the chain up. See the issue is you can't wrap it around your hands to make the area smaller so there is too much slack. Can be easier to get away, but with injury. A regular chain would be better because you could wrap it around your hands to tighten it. Now it could maybe be ok for strangling a giant. Remember, spike chains are cool and all but (and I am thinking 3.5 here) there are two rings that are safe hand holds. You can't touch the rest of it without hurting yourself so it is kinda limited in what it can do by the sheer fact that you can not adjust your hand placement.
I think about alternative uses for weapons and how to take down enemies a bit too much. I'm not allowed to interrogate people in our games anymore.
