He and Irene and Portia, all more and more piano [quieter, lower in tone and voice], trailed up and down the cold parts of Riviera, till he caught a chill and died in a nursing home.
Piano [Oxford English Dictionary]
1. a. adj. Of the expression: Soft, low (also fig. gentle, mild, weak). b. adv. Softly, in a low tone or voice. Abbrev. p.
1683 Purcell Sonnatas in 3 Parts Pref., The English Practitioner..will find a few terms of Art perhaps unusual to him, the chief of which are..Piano. 1724 Short Explic. For. Wds. in Mus. Bks. , Piano, or the Letter P, signifies Soft or Low. 1762 Colman Musical Lady i. 11 O Piano, my dear Lady Scrape, Piano. a 1817 Jane Austen Persuasion (1818) IV. vi. 120 James Benwick is rather too piano for me. 1856 Mrs. C. Clarke tr. Berlioz' Instrument. 5 Chords of three or four notes..produce rather a bad effect when played piano. 1884 Blackw. Mag. Dec. 782/2 The cry for peace will probably become very piano. 1886 E. L. Bynner A. Surriage xiv. 157 The music lapsed from piano to pianissimo. 1900 E. Glyn Visits of Elizabeth 188 The Marquis..looked thoroughly worn out and as piano as a beaten dog. 1922 A. Huxley Let. 9 Sept. (1969) 209 Aunt Nettie is with us: but happily she is in a very calm and piano mood so that she is quite an agreeable companion. 1941 [see exalté a.]. 1953 E. M. Forster Hill of Devi 138 Very piano and tired, poor dear.
2) a scrap of a widow [Oxford Talking Dictionary]
The important phrase is 'a scrap of'. To answer your question, the meaning is ambiguous:
Option a) The widow is a (very) short in height.
Unlikely option b) The lady is a very frail widow. She is so sapped that she isn't almost a widow anymore. She may look like she is passing away soon, as she seems so weak.
She was a scrap of [= a very short] widow, ever so plucky, just back from china, with damp little hands, husky voice and defective tear-ducts that gave her eyes always ratehr a swimmy look.
Oxford English Dictionary
d. A small person. colloq.
1898 H. James Two Magics 60 ‘Perhaps she likes it!’ ‘Likes such things—a scrap of an infant!’ 1928 E. P. Oppenheim Chron. Melhampton v. 146, I wasn't here for long, and I was a scrap of a fellow those days. 1939 N. Streatfeild Luke 109, I didn't know the poor little scrap could look so radiant. 1958 Woman's Jrnl. Mar. 77/2 ‘The woman?’.. ‘They picked her up late last night. Poor little scrap.’
3) prostrated [Oxford Advanced Learners Dictionary]
She had a prostrated [having lost all strength or all determination because of an illness or an extremely bad experience] way of looking up at you, and that fluffy, bird's nesty hair that hairpins get lost in.