How dangerous is it to go down on a provider who seems very clean, is new to the hobby, and says that she is safe? What protection should be used, if any?
First, please read past posts on this very topic.
Second, it is up to you if you wish to DATY or not. If this is something you'd like to do, but are unsure of the "cleanliness" of the provider use a dental dam or saran wrap.
Personally I have never had a problem, but the risk is there and that is for you to decide. And there are dental dams, but who wants to lick saran wrap? Many discussions on this topic, but in the end it is whatever you are comfortable with.
See the link to a factsheet on different sexual activities and associated risks.
You can use a dental dam or saran wrap as others have suggested if you don't want to take the risk.