I don't think it helps to just send people to the other forums if they want to talk about purging, since that may be something that is part of their anorexia if they're b/p subtype.
Sometimes if the person has an anorexic mindset, it's hard to relate to the people in the bulimia forum.
I know this because when I used to b/p and restrict, I would've technically fallen under the anorexia b/p subtype because of the nature of my ED, however at the time I felt I related to the bulimia threads more. My experience was the opposite, but it applies to someone who has anorexia b/p type. Just because they purge, doesn't mean they can relate to bulimic topics, just like just because someone restricts while purging it doesn't mean they relate to the anorexia topics.
I hope that made sense?