I go with a variation on Lynn's, "She was mean--" she dragged 'mean' out to at least five syllables "--and I told her to shut up," she said.
I also like the inference of character in the statement, "she dragged 'mean' out..." etc.
In dialogue it is common to use italics to indicate a change of tone, in this case being useful to tip the reader to the interjection: "She was mean--" she dragged 'mean' out to at least...' etc.
I think the ambiguity is aggravated by two people being referred to as She, she, I, her, she in the same sentence. I'd make it two sentences, using the speaker's name in attribution. "She was mean...," she dragged 'mean' out to five syllables. "So I told her to shut up," Andrea said.
I know I'm new here, but I really like this kind of thing. I'll have a query letter up in QL Hell in a few days; I hope you'll take the time to help me out with my questions.