What a sad story.Your friend has to do what he feels is right for both birds.There is no reason why he shouldnt take the grey in, but it may not be simple as the grey is old & will be attatched to the current owner & set in his ways.If it was me i would consider visiting the grey to see if he could establish a friendship & let the grey become fimiliar with your friend,If possible could your friend take his grey for a visit to,keeping them in seperate cages etc..see what reactions each grey has.
Both greys could live happily in seperate cages,after time they may be let out together with close supervision on the other hand they may not get on & will have to come out at seperate times.It really is a lot to consider & how much time your friend has to give both birds.