Underweight/neglected horse on our farm what are our rights?

We have a new boarder on our farm her horse is underweight and has been for the month its been here. The owner hasn't had hay in over a week or grain for a few days. Now of course we are feeding it (we could never let a horse go hungry) but this woman is sue happy, i think we should give him different food and/or beet pulp does anyone know if we have the right to add to a horses diet that doesn't belong to us? Please help! also he has a cut that they have not been caring for, yesterday we unwrapped it to take care of it (we feel so bad for this guy and cant just leave him go....) that is now maggot infested can we call a vet and bill her??