For 99% of people who have never been published before, you need to have your novel completed before anyone will look at it. For non-fiction you can sell the book before it's written. If you've been successfully published before, a publisher might buy it before it's complete. But this isn't going to happen for someone who's never been published before, regardless of what your friend says. And, you don't have to listen to me or anyone else here, go ahead and send off an incomplete manuscript. Let your friend advise you. I'm not sure why you're asking us if you're friend is more reliable since she's already published. and it sounds like your friend has a non-fiction book (non-fiction is handled differently than fiction) or your friend has been published before and that helped her get the contract without having the book complete. If either situation is true for your friend, then you as an unpublished writer who's writing fiction cannot use her as an example as her situation is different than yours.
No, you don't need a literary agent to be published. Chances are, though, that you're only going to be published by a small publisher (some of which are very well respected). Or maybe you can attend a writers' seminar or convention where you can pitch directly to editors and get lucky enough to find an editor at a big punishing house in which to pitch (they might invite you to submit your work within a limited window) and you won't need an agent - if they make you an offer, you can get an agent or a publishing lawyer before you sign anything to help negotiate your contract.
And, you talk about your friend being published, wouldn't she be the best person to ask about all of this? Wouldn't she know how to get an agent and where/how to look for one? I know I certainly wouldn't value the opinion of complete strangers over a friend who is already published and apparently knows the business, whereas you know absolutely nothing about us strangers and whether or not we're giving you bad information.