meaning in context - Equivalent AmEnglish expression of BrEnglish slang term "cheeky"

Norman Schur, Eugene Ehrlich & Richard Ehrlich, British English A to Zed, third edition (2007) has this entry for cheeky:

cheeky, adj. Very impudent and disrespectful in speech or behavior.

But impudent in the sense of "cocky boldness" (as Merriam-Webster's Eleventh Collegiate Dictionary puts it) isn't necessarily a simple matter of regrettable rudeness or disrespectfulness, and I can imagine cheeky, in this same sense, implying (as Rick Deckard suggests in another answer) a kind of free-spirited sassiness that is actually charming and even admirable.

So perhaps either sassy or impudent might convey the sense that your British friend had in mind.