You could try Solid Gold's Holistique Blendz. That's an allergy food really, but it's also low protein at about 18%, and very low fat (6%). That's lower than you'll find in most others.
Another possibility is California Natural's lamb/rice formula. It's a little higher in protein at 21% - but it's also a very good food. With such simple ingredients (it is essentially lamb, rice and sunflower oil), it may be easier on the liver than just about anything else (there is also a chicken version, but it is a little higher in protein). And there's a reduced calorie version of the food that has just 18% protein too...
I don't know that much about liver issues, but I do know that the old theory of low protein for kidney issues (same reason - to reduce stress on that organ as far as possible) is being challenged in the veterinary field. Many people believe that it is not the amount of protein that's the issue, but rather the quality - high quality protein being far easier to digest than low quality. Accordingly, the aim is to use a good quality food as can be found that has plenty of high quality meat protein (which is easy to digest) and as little plant protein (which is difficult to process) as possible. Makes a lot of sense
and that's something you might like to bear in mind in choosing a food for your girl.
Personally, I think I'd go with the Cal Natural - it may have slightly more protein than the Solid Gold, but it's higher quality protein and the ultra-simple ingredient list should help to allieviate any stresses caused by difficult to digest grains (rice is about the most easily digested grain you can get - it's still grain, but it's easier than most others).