Getting curls back

My girlfriend has natural curly hair. She has been straightening her hair quite often and now she's starting to lose her curls.

I see exactly what you mean. I had abused my hair so much that my natural curls are so hard to wake up!
One think I will suggest is to stay away from silicone base products, this will make hair worst! Read the labels for the ingredients, use the internet to research what ingredients are for.
Remember most  "Curly Hair" products normally target girls with soft hair texture.
This is what I am doing and finally my hair is getting back on track:
1. When you shampoo, use a 2 in one shampo/conditioner for DRY/Damage Hair.
2. Condition, Condition, Condition. Use a creamy conditioner for daily use  (made for dry/damage hair) Twice a week I do some deep keratin conditioner  to reconstruct my hair.
3. Use a Leave in Conditioner. Fructis Curl & Shine is the one I've been using until I find a better one.
4. Apply a curl enhancing cream/lotion/gel and let it air dray.
5. Stay away from the heat for a looooong time. Let the curls come back to life.
I would love to recommend a product for your friend's hair but I am still in the search for the perfect one.  A lot of research has been helping me on trying various products, some with no success, others with great success for one day. Once I continue using them it seems the problem get worst or my hair wont like them.

I started making a journal to document my hair changes, and for motivational purposes. I think every girl going through the same hair issues should make a journal, write it down or something. Get to know your hair. And mostly learn to love your hair.

I will post some hair products later that worked for me. If you want to see my progress I invite you to look into my online journal. I only wish I started doing this a long time ago because I have come very far on my researches. My hair now is nothing compare to my hair a month and a half ago. YES, that fast I am seeing some great progress.