Yeah, 5yrs old is too young for the HP-series, but I guess the mother didn't think about that beforehand x).
My daughter is five (coming on to six) and I have read her the first two harry potter books, something which she really enjoyed. As with all "age" things I don't believe there is a magic number, it will really depend on the child.
Why Harry Potter? Quite simply because we have been playing the lego game together (another great experience to share). Naturally she was interested, saw the books on the shelf and wanted to read them herself. Whilst she is quite capable of reading them, I wanted to control the situation a bit to make sure she understood exactly what was going on.
I didn't censor anything really, I just explained certain things. The big problem in the first two books is dealing with the death of Harry's parents. There isn't a lot else that she isn't beginning to face in school and in this regard it was quite helpful.
The lion king already brought up the death subject (most disney movies are messed up in some way) so it is good to be able to explore it with her a little in a different context.
I'm not going to move into the third book because I think a lot of the themes are not quite as relevant or interesting and there are other things for her to read. But the fact that she associates so strongly with Hermione and will randomly case "Wingardium Leviosa" (or whatever it is) on me just makes the whole thing worthwhile.