After moving to Vienna, Ignaz Moscheles became good friends with Beethoven, even making piano arrangements of many of his compositions, including Fidelio.
Moscheles was a composer and pianist from Prague who came to Vienna in 1808.
He was at the concert on 22nd December that year, at which Beethoven premiered the Fifth and Sixth Symphonies, and has left an eye-witness account of how the Choral Fantasia came off the rails.
"During the last movement of the Fantasia I perceived that, like a runaway carriage going downhill, an overturn was inevitable....." He was in frequent contact with Beethoven in Vienna between 1808 and 1820.
In 1814 he made a piano reduction of Fidelio, which Beethoven corrected, and later made piano arrangements of many of Beethoven's compositions.
He moved to London in 1825, but Beethoven stayed in touch with him, writing to him three times during his final illness.