An RSS feed is not the same thing as an email update subscription – as you seem to be finding out. What both of you seem to want is a way to get an email update for any new posting to the forum (or perhaps to the entire site). Apparently nobody in the WordPress ecosystem has either done this – or if they have, they have not shared it.
While it would be possible to redirect the RSS feeds to feedburner, that is a free service now owned by Google and now, for a variety of reasons, feedburner is not expected to be available much longer.
What we all apparently really, really want is a more global “Subscribe” capability, and surprisingly I am just not finding it. In fact, I have found out through experimentation that subscribing to a forum category is surprisingly limited in that it does not appears to include sub-categories. The only WordPress plugin I have found would require me to manually subscribe all who are to receive email concerning all updates, and that is just not scalable at all.
In the meantime you could use Firefox’s “Live Bookmarks”. Your tastes may vary, but I would not check Firefox’s “Always use Live Bookmarks to subscribe to feeds.” because I see user comments about that being a very difficult decision to undo. I have no experience at all with any of this RSS feed world, in spite of having daily frequented blogs and forums for decades.