Ever since Twitter hid their RSS feeds, people have been having to guess at the addresses. The Twitter API has several different ways to reach the same data so here’s a user ID and RSS feed fetching tool.
All you have to do is go to http://zoril.co.uk/twid, type in their user name, and hit lookup.
You’ll need javascript enabled as it means that the requests will come from you rather than this server, which saves me hitting the request limit. As such you can view the source to see how it works.