OP I do think you are being a bit harsh on the other owner. You didn't exactly give her much notice to get her wormers sorted. If you don't want your wormer to have been wasted I recommend you pop down to a farm supplier and get some diatomatious earth ( food grade not the red stuff that's been treated its poisonous to horses) and start feeding it daily to your horses. It will help prevent reinfestation. Brilliant stuff I now use it in place of chemical worming and all my worm counts have been excellent.