Yes he's correct. It's 1 bag per passenger max 15 kgs. The max limit is applicable for both Y & J passengers. So if you try and take a 20 kg bag... you will be at the mercy of the check-in staff to allow you through. By the rules they can make you pay for the 5 extra kgs! These rules will be very clearly stated in your ticket and on the 9W website.
With the current state of 9W.... I would not take that chance... they'll be looking to make money every opportunity they can right now!
And welcome to FT.