My house probably looks similar to what you describe, OP. Which is why I caution about books. Because my kids have millions of toys...but they have even MORE books. I cannot turn down a request from my child for vegetables, exercise or a book!
I like the idea of experiences. Does your friend have easy access to 1:1 time with each child where another adult takes the other child? If so, watch Groupon for deals on a sample Mommy and Me type class. Art studios, gymnastics places, yoga studios, indoor soccer, nail painting places, laser tag.
Or find out her big gift and get the cool accessories. A bike helmet shaped like a unicorn or bike bell or basket is an awesome present for a child who just got a new bike. A personalized chef's apron and hat is perfect for a child who just got a toy kitchen. A conductor's hat and train whistle for a child who just got a train table. My DD's grandmother got her a purple gerbil wheel for her birthday to go with her new gerbils, and she was ecstatic.
But really, you may also need to set your expectations. It doesn't sound like your friend is teaching these children to be gracious about gifts or appreciate the kind gesture of a gift. That is unfortunate, but improving the relevance of the gift is not going to resolve that.