No, you can't re-use the soaking water to soak again. The whole point of soaking is to leach out the sugars. So the sugar is in the soaking water. If you soak dry hay in sugar water, it won't leach out any more. Also, the soaking water may ferment, which is potentially dangerous.
Please tell whoever asked that they should drain the soaking water where horses can't drink it.
Maxine and Indy (PPID) and Dangles (PPID)
Canberra, Australia 2010