I have a friend with an idea that will help Huggies or Pampers improve
their product and sell more diapers.  She has not told me the idea
yet, but assures me that the idea is excellent and will make them
better/more convenient for Mothers.

As this is a massive market, I know an improvement like this could
mean a ton of value to the Diaper manufacturer/marketer but am not
sure how she can approach them with this idea and ensure she gets
paid accordingly if it truly does create value for them.

Who to contact?
How to approach them to ensure she gets compensated if they use the idea.

Thanks

Dear Kevin, 

First, the bad news: Even if I gave you the contact details to R&D
managers at these companies, the chances that they'll listen are
small. Even if they agree to meet her and listen, they might not take
her seriously, if all she's got is an idea. And finally, if they do
take her seriously, the mere fact that they see that they deal with an
"amature" might lure them to take the idea (if it's good) without
giving her any credit (or money) for it.

In other words, you have any reason to believe that your idea -
contacting these people directly - might crash her project. The only
way to ensure that your friend's idea will not be "stolen" by these
companies, and to optimise also your chances of being taken seriously
and the idea bought by one of these firms, is to register it as a
patent.

Here are some patent authorities: 
United States Patent and Trademark Office
http://www.uspto.gov/

European Patent Authority
http://www.european-patent-office.org/index.en.php

The UK Patent Office
http://www.patent.gov.uk/ 

Your friend might need some help in registering the patent. Here is a
couple of  companies that provides this service:

Invent-Tech 
http://invent-tech.tv/inventions/?sc=Google&key=idea%20Patent&frm=Mas-100&ad=Patent&disp=searchURL

Davidson Inventegration 
<http://www.davison54.com/?function=11&source=64> - also offers a free
booklet on the subject.

Idea Trade Network
<http://www.newideatrade.com/> 
"Idea Trade Network (ITN) is an award-winning one-stop global forum
that allows companies and individuals to buy or sell ideas, new
inventions and patents as well as art, crafts   and other intellectual
properties."

She can also check out the following advice books, before she goes to
professionals:

Patent It Yourself / by David Pressman

How to License Your Million Dollar Idea: Everything You Need To Know
To Turn a Simple Idea into a Million Dollar Payday, 2nd Edition / by
Harvey Reese

Will It Sell? How to Determine If Your Invention Is Profitably
Marketable (Before Wasting Money on a Patent) by James E. White

The Inventor's Bible: How to Market and License Your Brilliant Ideas
by Ronald Louis, Sr. Docie

The Complete Idiot's Guide(R) to Cashing in On Your Inventions
by Richard Levy

Try also : 
Inventors @ About.com
<http://inventors.about.com/od/fundinglicensingmarketing/> 

Andy Gibbs "Side Dish: How do I turn my invention into a profitable
part-time business? Our Inventions Marketing Expert offers these
tips.",  Entrepreneur.com, June 05, 2000,
<http://www.entrepreneur.com/article/0,4621,275660,00.html>

I hope this answers your question. Please contact me if you need any
clarification on this answer before you rate it. My search terms were:
"how to sell" idea patent, "how to present" idea patent, patent