What that woman went through is probably one of the most horrible, painful and traumatic experiences one can survive. And, from what you say, she helped your daughter escape the same. She literally saved your child. Can you match this for her? No. Can you pay her back? No. Can you 'just' acknowledge? Yes.
As someone who has saved lives (once decades ago, then a few years back), I can tell you that the greatest reward you can get is just the memory of doing it. No matter what people told me, the praise I received (or not...), it didn't matter. My biggest reward was the relief I felt and pride I remember (and still feel sometimes) and that I never expressed (or needed to). I sincerely hope that the memories of her actions will periodically resurface, and make her proud and heal.
How can I express my gratitude without sounding indelicate about what she went through?
You can read about what you can do, and what you should not do, for instance: supporting a survivor or help after rape and sexual assault. A quick search will offer lots of helpful reading, such as this one about PTSD trauma: recovering from rape and sexual trauma.
- Listen to the person, but don't ask anything about what happened.
- Offer practical support if possible/needed.
- Respect their decisions, and their yes/no.
- Bear in mind they might not want to be touched or even approached. Stay a few feet away.
- Don't forget that it will take time for them to deal with their feelings and emotions.
What I usually do, when facing someone who's mourning for any reason, or has been through very hard times, is just offering support: "if you need... feel free to call/ask...". Never say you know/understand what they feel. You don't. No one, but she, knows.
In your case, I would just tell her how great she's been, and still is. How 'enormous' her impact has been on your life and your daughter's. Point out her actions, avoiding carefully what she suffered while doing it or because of it. Don't bring back awful memories, but express the positive of her actions, of her courage, of her strength. I've once been thanked in a very simple way, and that was more than enough. The people just said: "Oh thank you! Thank you so much". Their tone said it all. The way they were cuddling the kid and reassuring him was enough to show love and relief. Love. Relief. Happiness. 3 words for a great action and a great reward. Were I to thank someone like her, I would just try to express the same. Very few words, but very powerful words. Or show her. Still shocked and recovering, she may not be able to fully understand how much she's helped both of you, but she may see it, and remember it in the future: a dad and his daughter together in front of her, holding hands or cuddling thanks to her courage. This may be enough, as, sometimes, an image is worth thousands of words. I wouldn't say too much.
FWIW, a last note: be seen as a dad, not as a man, if you think it can help her. And if it happens that you meet her, make sure your daughter is with you, or some of her family.