If she liked Cold Comfort Farm, Virago have republished Nightingale Wood.
They are publishing a lot of books by Rumer Godden and some by Barbara Comyns next year (I know, you want something now!)
Trying to remember what I liked at her age.
I liked Cat's Eye by Atwood when I was young, but you say she's already read a few.
Mary McCarthy, The Group (VMC)
Jessica Mitford's memoir Hons and Rebels
I liked Dubliners when I was 18, not Portrait of the Artist as a Young Man
Edna O'Brien's Country Girls trilogy or just the first one
Colette, Claudine novels and short stories
Winifred Holtby, South Riding (VMC and other editions, recent TV drama)
Elizabeth Taylor, Palladian (VMC)
Short stories - Katherine Mansfield, Mavis Gallant, Anne Tyler
Emile Zola - Nana and maybe L'Assommoir, The Ladies' Paradise (recent TV drama)
Antonia White, Frost in May (VMC)
Edith Wharton, The Age of Innocence and The House of Mirth (VMC)
Sybille Bedford
Willa Cather, My Antonia, The Song of the Lark (VMC)
Janet Frame, An Angel at My Table
Florence King, Confessions of a Failed Southern Lady (I might well have been 17 when I first read it)
Muriel Spark