If was only half eaten then it was something else. Maybe it is the molt or it just died and the rest of the CUC hasn't found it yet.
What was the LFS
feeding it and did you get a chance to see it eat? Did they say only feed once a week? From what I have read on ribbon eels they can be rather difficult to keep long term. There is a lot of information on them but trying to decide what is actually needed for them is difficult except they do need life food usually fish and some shrimp. Trying to wean them off live food is hit or miss as well.
They are nocturnal as well. You will be lucky to see it during the day much.