Because you asked this question, I imagine your girlfriend's status as a fatherless daughter is causing her pain and damaging the relationship. Please be aware that it's extremely difficult for a woman to get over the rejection of her father and many never do. Some cling to it as a central part of their identity. Without it, they would have to take responsibility for their own lives instead of blaming their dads and that's much harder and scarier.
This is a crucial moment in your relationship when you should step back and ask: Is she really motivated to change? If the answer is “yes,” then encourage her to seek help from a professional. Going into therapy is the single best thing that she can do to grieve her dad's absence and to learn how to move forward from it. If she's not willing to do that, you should consider ending the romantic part of your relationship. You could remain friends, but she would not be a healthy candidate as a partner, wife, and mother.
If she chooses therapy, there are many ways you can be supportive. Encourage her to talk about what she's learning and write about it in a journal. Do activities together that promote her overall well-being such as spending time in nature, taking long walks together, cooking, and meditating. Volunteering in your community is also an effective way to help her move away from her own suffering. Most importantly, encourage her to “feel the feels” about her dad's rejection. Bottling up her emotions or numbing them with food, drug, or alcohol can lead to depression.
You sound like a caring boyfriend and I hope your girlfriend will seek professional help. I know you want what's best for her, but she must want it as well. I wish you two the best.