If you accidentally sent Billy with the hookshot then change the control over to Rebecca and backtrack all the way to the mansion, more specifically in the area where you had to light the candle a little bit earlier with Billy. There should be a bedroom nearby; have Rebecca look around in there and she'll find the green capsule. Check it to find the green chemical, then head back to Billy and change the control over to him. Raise the wall there with the switch then have Rebecca kill the leech zombie as quickly as possible. Now you can enter this room and examine the red capsule with Rebecca to find the necessary item. You're all set.