If you are the only other nurse on the unit, then you are in a tough spot to try and refuse to cosign.
Again, you are only witnessing that the UNUSED portion of a med is discarded appropriately; not as witness to where the rest of that med goes. Be sure when you sign as witness, you INSIST the drug be wasted at that time, in your presence. If the nurse declines to do that, then I would state to them that I cannot cosign the waste.
Tough spot to be in, for sure. Just make sure YOUR medication handling is spotless. You cannot control someone else's practice. Sounds like they are digging their own hole, for sure.