My guess is:
Nevertheless, I don't know where this puzzle comes from but that won't be surprising you are right and your friend may have made a mistake... Unless PSE community finds a simplier solution :)
Whenever I see a question like this, I imagine there is unlimited formulas to cover each optional (randomly) selection of those numbers in any direction, and if there isn't any other limiting rules, every answer to that question is correct.
take a look at this