There's various versions of that story. Some say that he was so involved that he, at the very least, should have a co-author credit. Others say it was more in a guiding type/"here are some notes" type role but that the bulk of the work was hers. Those that ascribe to the "Capote actually wrote it!" theory point out to the lack of other published work from Lee. Of course, I've always held that if your first book is "To Kill a Mockingbird," you may as well just hang it up because you're not going to top it.
There is private correspondence from Capote to others saying that Lee was working on a book, and there are a lot of notes between Lee and her editor that seem to refute the idea that he was the author. Capote's father was one of the big proponents of that claim, but in the end there's not a ton to support it. All we really know is that Lee and Capote were close friends and it would be reasonable to assume he, at the very least, gave her some feedback along the way. Whether it was more or less, who can really say.