What guidance is there for pilots when reporting braking action?
The FAA issued a Safety Alert for Operators (SAFO), "Runway Assessment and Condition Reporting" #16009 on August 15, 2016. In it, it says that:



You'll notice it doesn't say "Fair" anywhere on there, that is because this SAFO replaced the word "Fair" with "Medium" on October 1, 2016. 


  Pilot braking action reports will continue to be solicited and will be used in assessing braking performance. Effective October 1, 2016, the terminology ”Fair” will be replaced by “Medium” and pilot braking action reports will now describe conditions as Good, Good to Medium, Medium, Medium to Poor, or NIL. This will harmonize the NAS with ICAO standards.


(Emphasis mine)



Since the accident with Mike Pence happened on October 27, 2016, if a pilot was reporting "Fair", they were in violation of the SAFO and should have been reporting "Medium". You can read the NTSB report of this incident here.