[_ck_]

I suspect the problem is they have a link to bbPress’s http url from a page that is already https. So the browser gives a warning that the visitor is traveling outside of https.

You could install bbpress as normal to the http url but link from that page as https and just rewrite the url.

Offhand, if you really have a SSL cert though, the next version of bbPress, like WP 2.6 will be able to handle https logins for what that’s worth (even the current version could be forced to with with SSL but it’s overkill).