I believe that Content Editor users can use HIPO Herder, it just isn’t linked in the usual spot (or anywhere, it seems).
As far as I can tell the only link to HIPO Herder for Content Editor users is when they try to log out with a stuck HIPO - it warns them about the stuck job and provides a link to HIPO herder.
So you can either tell them to try to logout, or just get them to bookmark /_admin/?backend_section=hipo_herder to access HIPO Herder directly. Once there, they can only see their own HIPOs, not those belonging to other users, and resumption seems to work fine.