I'd see that as two separate sentences that just got run together as the person serving is really busy and trying to get through things fast like a production line. The general impression being:
"Can I help?" [standard 'meaningless' greeting to the customer but rattled off because it is allegedly 'polite'] + "Who's next?" [who is next in line?]
That is how I would interpret it, rather as "Can I help [the person] who's next?"