What learning resources are there for part-time DBAs?
If your going to be "The DBA" (part or full time) you should learn as much as you can before the former DBA leaves. I think people in this situation should talk to anyone who knows about the database (the data flow, the design choices, common task, etc.), it is very hard to try to learn the db even if documentation exists.

Also, the person should have something like a dba profile, If you don't like it you would be doing something that brings no excitement and you will have no satisfaction from your work.