Building any forum is a challenge, and a company one is a unique challenge. In no particular order, here are a few thoughts:
1) Get at least one or two senior managers to participate. This will be a big encouragement for others to spend a little time there.
2) The forum moderator or admin needs to "fan the flames" of conversation. If someone posts a question, your wife should ensure it gets answered, even if it means phoning up someone else in the company and asking them to reply.
3) Some kind of off-topic or social forum is probably a good way to engage some users. If they know there's a discussion about, say, what games to play at the company picnic, they may check back more often.
4) As in any community building role, your wife can start some topics designed to provoke discussion. A news story in the WSJ, a complaint letter from a customer, etc. can be good thread starters.
On the plus side, she doesn't have to worry about SEO, generating outside traffic, spam prevention, etc. And, in this setting, I'd expect everyone to behave well.
Good luck to her, Status_203, and encourage her to become a member here to share her experiences!