I had to revisit the http://www.flyertalk.com/forum/23593103-post6.html thread to check up on IATA’s Baggage Rule Resolution 302 which standardised baggage policies for passengers flying multiple carriers. Here it says that if the baggage provisions of the different carriers are not the same, the policies of the Most Significant Carrier (MSC) will apply. The MSC is defined as the first operating carrier crossing IATA Areas or Sub-areas.

So in the case of your daughter and son-in-law’s booking. my reading is that the outbound sector in WT+ should have used to calculate the baggage allowance across the entire trip.