I have had a couple of biters, one of which started biting very frequently at about 15 months. I worked with his parents on a consistent plan of time-outs and teaching gentle touching. But what helped the most was when we started giving him his soother all day long instead of just at naptime...it was out of desparation as we had tried a teething ring attached to his shirt and that didn't work. Next step would have been termination and parents had been warned, but the soother worked! he only needed it for a couple of weeks and then we weaned him off of it and he rarely bit after that. Having the soother in his mouth prevented him from biting and gave him the oral stimulation he needed. If the child takes a soother at all, I would suggest it. Soothers are not ideal, but better that than having kids go home with bite marks. If the parents are not working on it with you then I would just terminate as it is unlikely to improve.