I use Active Forums for one of my clients and I can't say enough good about it. It was running for about a year and half without so much as a twitch until someone hacked the site and we had a total do-over. (hack not forum related.)
It sends custom notifications of when posts are made, or you can watch conversations. You can set all sorts of security and features down to the discussion level. It does just about anything you would want a forum to do. It even lets you send mass mailings to users.
last time I looked at the phpBB it lacked the features of better forums and was difficult to set up. of course that has been along time and it may be better now but it would have to go a long way to beat Active Forums.
I have never found PHP goodies to work with DNN, but that is just my experience. I agree with Mitch, get them to switch to Active forums. It isn't expensive and well worth the cost. As for migration, AF has a migration tool built in, that installs like a module, but I don't know if it will go off the site to pull posts elsewhere. Ask the folks at AF and they may be able to help you with that issue.
Beth Vest
DotComDominion.com
http://www.FPTV17.com