Hello,
If your friend doesn't already know about EFT, perhaps you could introduce it to her and see what she thinks. You can also talk about the ideas you have - anger, rage, helplessness - to give her ideas for tapping.
If you'd prefer to surrogate tap for her, the key is to be open to whatever messages come your way. So I suppose a good starting place would be, "Even though I have this rage...." and keep tapping and seeing where that takes you. You don't really have to rephrase in a special way; you can use the tapping phrases as if you were the person themselves.