What does Higgins say he could do with the flower girl? What will keep her in the gutter to the end of her days? (Act 1)

Higgins says that if he works with her her can improve her English to the point that she'd pass for a duchess. Her "kerbstone" English would keep her in the gutter.

THE NOTE TAKER. You see this creature with her kerbstone English: the English that will keep her in the gutter to the end of her days. Well, sir, in three months I could pass that girl off as a duchess at an ambassador's garden party. I could even get her a place as lady's maid or shop assistant, which requires better English. That's the sort of thing I do for commercial millionaires. And on the profits of it I do genuine scientific work in phonetics, and a little as a poet on Miltonic lines.