I've known a few people with cancer who had a problem with getting enough of this.
A loss of appetite is often a result of chemotherapy. It's hard to see to it your loved one gets these building blocks when they can't eat.
I spent a lot of time making chicken livers as a kind of natural supplement. I had a good recipe and it went down well.
They make fortified drink mixes packed with essential vitamins and such, but sometimes the flavor isn't great.
I had some luck with breakfast drinks. They make those drinks you can replace for a breakfast meal. You mix it with milk, so that is a plus there as well.