If it was booked as one ticket by a carrier ie Air Seychelles or EK then whoever booked and added the connecting segment should be able to assist with food and a hotel until either EK resume the flight or they can look to reroute.
If they booked through a TA or a diy then they may be stuck inbetween as Air Seychelles may say it's done it's job and may not be willing to return and EK may say until we can resume flights we can't do much.
They can try going into DXB as a tourist but not sure if Seychelles is a free visa on arrival location. If they can't leave and assuming they have travel insurance I would suggest trying to claim but staying in the airport hotel in T3. Not exacally cheap but atleast they get a better bed. It would be good to know the reason they have been told can't leave airport, sometimes your nationality may have an influence with the UAE and they may think your not able to support or abscond. The longer he camps the more pressure they will put on either EK or someone to either move on or assist on returning to origin,
I guess questions is what nationality is he and was he visiting as a tourist or national returning home.