The problem with the Duo Rad is if he has large cast iron upstairs he
will have uneven heat, sure it will get warm but cast iron will radiate
heat for an hour, the Duo Rad stops when the pump and fan shut off. Best
is the same radiators as he has upstairs and proper btu output. Mix
output types and often a new loop , pump and thermostat are needed to
achieve a balance.
I have cast iron, someone remodeled a room took out the cast iron and
put in units similar to Duo Rad. Heat was bad always till we put back in
the original cast iron. Mix and you can Mess up. That is where a pro is
good to figure it right the first time.