Depends on the person. If she’s a student of logic and you have a good relationship with her, suggesting she committed a falacy won’t upset her. She’ll probably laugh and say, “Yeah, you’re right! Let me try again.”
If the person isn’t familiar with fallacies calling her out for them won’t help, but instead of saying, “You’re committing a straw man fallacy” you could say, “You make a goodnight point! But it seems I did a poor job of communicating and you misunderstood the argument I was making. Sorry for being clear as mud! Please let me try again.” That way you’re saying you’re the one to blame, not just her, so she doesn’t feel like ahe’s being put on the defensive.
I also find that asking questions (the Socratic Method) is a great way to keep a conversation from getting heated.
Tone and attitude is the key. If you’re being kind and respectful and showing you’re really interested in listening and finding the truth together, and you’re not interested in getting to your next point and trying to win some debate, you’ll reach her with your behavior as well as your words.
Make sense?