You’re a good friend in wanting to help her, and you’re correct in thinking that therapy is the way to go. No matter how carefully you choose your words, though, she might be offended by your suggestion. Sadly, there are so many folks who still see therapy as a stigma and a sign of weakness. However, you should recommend it anyway because it’s the right thing to do. Just be prepared for a negative reaction.
The best way to go about it is to use “I messages.” You could say something such as: “I’m concerned about you. I think that you haven’t fully dealt with the heartache that your father caused you. I believe that you deserve to get some help with that from a therapist.”
Of course, therapy will do no good unless the client is highly motivated to put in the hard work. It’s like taking piano lessons. The lesson itself provides a useful framework, but little progress gets made unless the student practices each day. A therapist can give a client activities to do and tools to use but, if they don’t follow through, it’s largely a waste of time.