Public Contacts folder to email address book problem

[soozle]

I've created a public folder containing contact eamil addresses. This appears in my "show names from" list when sending an email. Another user can view the folder in the folders view but it doesn't appear as an option when he's sending emails and the checkbox "Show this folder as an email address book" is greyed out. I've made this user an owner of the folder.
I'm using Outlook 2000 and he's using Outlook 98 (with Exchange Server 5.5 of course)
Can anyone help ?

TIA
soozle