Currently, there is no hardware being employed to resolve this issue.

I was hoping to do this with the firewall built-in to MacOS. My friend's current network gear does not support this type of filtering.

If it cannot be done with ipfw, I may need to compile ipfilter for him, or he'll need to reconsider upgrading his switch to something that supports this feature.

ipfw ships with MacOS 10.5 or better. This is why my post was in "OS X (Apple)" rather than networking.