I like Charlie. She's not being two-faced, she's venting so she doesn't blow up and say something she'd probably regret to Helen, upsetting her because she knows how emotional she is and causing tension in the camp.
I'm getting the impression that she doesn't dislike Helen at all, she's just getting irritated at the fact that she's not getting any food and that Helen barely even gives the trials a go before giving up. God knows I'd be frustrated too! She's just being human.