I think automatically in terms of tonic solfa, I can sing the tonic solfa notes of any tune straight off the top of my head, but I am amazed that any pianist uses this system. Is this the rule or the exception in Italy ?

It all depends whether he thinks in terms of relative notes which is what tonic solfa is all about, or whether he actually identifies each note on the stave with a corresponding tonic solfa note, so that for example doh=C, de =C#, ray=D, e=D#,mi=E,fah=F,fe=F#, soh=G,se=G#,lah=A, and ta=B flat.

Then you would just write the appropriate chord above the line, except you would use the solfa nomenclature.