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.

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.