I figured out the problem. It’s because the <id> field has been the same for all items for quite a while. His RSS system broke when his web host updated Perl and he’s been creating the feed manually since then. It looks like he just copies the <id> field from the previous entry. I let him know and he deleted the <id> field from all the entries and now it seems to be working.
Apparently NewsBlur requires that field to be unique even though it isn’t an official RSS 2.0 field. <guid> is an official field so it looks like NewsBlur interprets <id> as a synonym.