I actually would suggest *who* is correct here regardless of the declining existence of *whom*.  I parse the `who is next` as its own inner clause that is all collectively the object of `Can I help`.  A similar example might be:

> I know who shot him

Where the thing that is known is the collective clause `who shot him` rather than just the `who/whom`.  It would be rather odd to say:

>I know whom shot him.