In the good old days, when IATA ruled, there was an industry agreement on sharing the costs of transiting passengers. There was a four letter acronym for this service which completely escapes me now.
Basically the receiving Interline carrier would absorb the costs, and then bill the delivering carrier for their share. I think it was based on the prorate value of the through fare.
This deal didn't apply in, or to from some important geographic areas (the Americas). It also didn't apply on short haul routes like within Europe.
The original idea comes from the days when direct flights were simply not available and overnight layovers were required. These days I would guess that only a few airlines would still offer this, and only for on-line, whenever they need to do it to get the business.
The IATA agreement ended in the early 90s.
Have your friend ask the agent or his airline, you never know.