You've got a friend who is a real expert and you think they could help a lot of people by sharing their knowledge on wikiHow. Maybe you think they might get a real kick from helping other people out. Here's a plan to help get them over the hump to trying it out!

I don't feel like writing, and you can't make me! GRR!

[edit] Steps

Explain how easy it is to register and get started... and that you don't even have to register to jump in and write.
Many popular articles on wikiHow are viewed more than 500 times a month. Some pages are seen over one thousand! Explain how spending as little as 5-10 minutes of their time could benefit thousands of people a year.
Forward them an entry from the List of Requested Topics that you think they'd be able to write a great answer to. Explain how they'd be helping someone out with their effort. Note that if you click on the details link to the right of the entry, you will sometimes get a little more explanation, and also a page that can be easily forwarded.
As a last resort, bribe him or her, unless what he or she is going to write is "How to Resist Bribes."

[edit] Tips

If you can get them over that first wikiHow, hopefully it will be easy for them to see the benefits and get hooked.
Compliment them on something they are proud of, then try to get them to teach other people how to do it by...I don't know...posting on wikiHow?