She says one word.
"I'llkilljaimelannisterforyou."
I'm having trouble getting behind that. Regardless, that would be an interesting twist but it doesn't fit Brienne to betray Jaime like that. I'm thinking she said something to convince Stoneheart that she isn't loyal to Lannisters (as a whole).
However, how she does that with one word? Mad skillz, yo.