Okay, I am going to get a little nutrition geeky on you - forgive me. 

The best recommendation I can give anyone is to know how many calories her usual food has - the really meticulous of us geeks will say it is best to go by calories/kilogram but in reality, it is a lot simpler to go by calories/cup since most people in the US use a cup measurement.  Knowing how many calories she has been eating to maintain a healthy weight will help you determine how to adjust her food accordingly.  If you Google calories in beef/chicken, it will give you an easy to use calculator that will tell you the calories in various cuts of meat.  Then, you can just balance things out.  If she normally eats 500 calories of her usual food but she is only eating half her usual amount, you know you can give her about 250 calories of meat.  Dogs digest meat a lot more efficiently than grain, so depending on what kind of food you usually feed her, you might need to tweak things a little bit.  If she eats a grain-free food, you are probably fine to just adjust calories directly; if she eats a higher grain content (or a higher carb grain-free ) you might need to reduce calories on the meat level a little bit.  Another really good add-in that is simple to cook is eggs, they are one of the easiest to digest protein sources out there and a large egg is about 70 calories; cook it any way you want - scrambled, poached, fried (not in butter), over easy, "egg drop soup", soft boiled, whatever! - I give each of my dogs an egg every night with their dinner.

Some dogs are really good at self-regulating, most are not, so don't listen when seven seconds after she finishes breakfast she says she is starving and hasn't been fed ever in her entire life.  But, if she is acting more noticeably hungry than usual, things like green beans, peas, sweet potato, and pumpkin are all food you can give her that will help her feel more full without adding lots of calories.  I prefer frozen peas or green beans but if you do get canned, make sure to get no salt added.  You can get canned sweet potato or pumpkin - again, make sure the only ingredient is the sweet potato/pumpkin and it is not pie filling if you get canned - or you can get sweet potatoes and cook them for her yourself, whole pumpkins are hard to find this time of year but you can cook one of those up for her, too, if you can find them.

If you need more help figuring out the food stuff, let me know what kind of dry food you usually feed and how much and I would be happy to help you work out some ideas for her.

Okay, the geek-out is over, thank you for your patience.

I know you have talked about the pain meds she is taking but is she on any steroids as part of her cancer treatments?  That can make anybody super hungry and thirsty.  I saw in one of your other posts that you took off her fentanyl patch.  Boomer was very dysphoric (that's what the vets called it) on the fentanyl, so much so that I removed his patch about 36 hours after his surgery (on the vet's advice) - two full days ahead of schedule.  He had been rather restless and very vocal since he came home which is unusual for him, within 30 minutes of removing the patch he was sleeping quietly on his bed - it made a huge difference for him.

I would like to add my thanks for this post martia! Lots of good info.  I always struggle with calories and recently realized how much higher grain free food is in calories.  

Here in Uk kibble for adult maintenance should be 3900kcal per kg and most are. I am afraid that most pet food produced commercially follows a formula that is slightly different from the one used in the human food calculation meaning it is a bit more fattening than you think. Over a year it probably makes a difference and contributes to the canine obesity problem. If you have not come across the book by the US vet Dr Pitcairn then I would recommend it as he gives recipes for supplementing kibble. He advocates cooked meats and went head to head with Dr Tom Lonsdale the Australian vet who really started the RAW argument. I have a blackboard wallchart with dog name(five or six to feed)  energy requirement - grams of kibble - grams of nonkibble which is about double. To mix I do a bit of mental arithmetic. Sorry guys in US but using grams allows for very fine adjustments.  

Funnily enough my Jack Russell terriers are all nodding in agreement. One thought, coconut oil! We are getting into it over here on our rescue forum as we tend to have a lot of dogs who are underweight or in poor health. I read that it is not as fattening as some oils that you might add to feeds. We also have some anecdotal evidence for it healing and helping hair regrowth. Most research on it seems from the US. My dogs queue up for their little chunk when they hear the lid being unscrewed so might work as an appetiser in moderation.