The Lightning Thief question

It was blue with the outline of a dog on the cover. The title and author was in white cursive inside the dog shape, but the whole background was blue. I think the word "friend" was in the title somewhere. It was about this boy who hated his living situation with his uncle and couldn't wait to move out when he turned 16. He meets this girl named Halley (like the comet, she would add) who's super smart and loves math. Her dad is a magician/astrologer which the protagonist (Ben) thinks is super cool. Halley has a dog because she's a recovering cancer patient. They work together to train the dog who eventually ends up with a therapy dog license and they have visits at the library and it's super great and stuff. Halley invites Ben over for their Jewish new year celebration and recites a rap song she wrote about her best friend, who is Ben. Halley and Ben start writing a book together, with the two main characters representing the two of them. The girl representing Halley gets in a trapeze accident, so the two characters travel back in time through space to save her and get McDonald's. They end up helping some aliens with this weird box thing (in the story they're writing, not the actual book). SPOILER from here forward: As they're writing this part, Halley's cancer comes back and she gets really sick. Halley shares the last part of the story with Ben. The spaceship the characters are on with rip in half, and the character representing Ben saves the box, but has to sacrifice the character representing Halley in the process. Ben obviously doesn't like this ending, but Halley falls asleep before he can protest because she's so sick. Another SPOILER: Halley ends up dying the next day. This was a kids book but I legitimately loved it so much and I cried a ton after reading it. I'd love to recommend it to my friend but I don't remember the title for some reason so if anyone does it would mean so so much to me. Thanks!!!