I hear this all the time asked by cashiers, bank tellers and the like, and I'm wondering if it is grammatically correct. I'm not sure if they mean to say it like that or if they mean something like: "Can I help whose next?" which looks even worse to me. "Can I help the next person in line?" is awkward and longer, but would sound correct to my ears.
Can I help who's next?
I usually hear: "Can I help the next person?" or simply "Next, please". "Can I help who's next?" is probably just a shortened version of "Can I help whoever is next."