You may have already done this, but for completeness: First, view source on the feed and find the terms of use. If there aren't any (which is likely) then you'll have to guess.
IMO, what matters more than the law is not pissing off the site owner, so think about how they would view your app. Is it adding, subtracting, or stealing value? In general, you should be sure to preserve attribution, links, and any ads that are in the feed (if any). If your app is free, that's probably enough. If your app is paid, you should probably contact the site owner and offer them what you consider a reasonable revenue share.