Then, how is it that any one of YOUR sites get ANY new users, or new members?
If the person can't get visitors with a portal because search engines can't really slurp new content or crawl for new content, then perhaps its a better idea to change so that you can cater to the search engines. Which in turn means more new visitors, more new potential members, and unique visitors.
Once he gets a lot of new users, new visitors, and he can self-sustain his community without worrying about how users will find them... then he can go back to having a portal. Otherwise, if the portal can't be indexed the same way content is... Then this is a good idea.
Now, I'm not saying that portals can't be indexed, crawled, or slurp'ed. I'm just basing my comments on what people are saying, including in this thread. And with your quote; google thinks that your portal info can be seen as a duplicate to the one that you have on your forum which isn't accurate as per the whole portal ideals [xenPorta and vBadvanced comes to mind here]. Because the article isn't in two different places, it's in one place. Unless your software is vBulletin 4 - where you'd have to make a new thread that directs to CMS [posts] for integration. Systems like this is where Google is right. Google thinks it's two of the same article each time. So, you have 1,000 CMS articles, and 1,000 of the same content [where the actual replies are], it turns into 1,000 duplicates. This is where googlebots gets confused on which link is the actual article.