someone who takes both sides in a diplomatic confrontation.
Two people get into a disagreement. A third person tries to make peace. The third person, the fence rider, will speak good or bad things towards each of the two keeping everything equal. See both sides of the argument. Sometimes used in a negative sense, meaning that you can not depend on this person to support your argument.
Someone who will say anything to avoid rocking the boat. They will go against their own morals and standards to make statements to please others. They shouldn't be trusted and confiding in them isn't a good idea. They are liked by many due to the fact they always have good things to say, but are not trusted by the smart ones who have figured them out. Fence riders are phony butt kissers. We all know a few, most are politicians.
Wow!..You look great in that dress!
OMG!..Doesn't she look awful in that dress!
It's not a matter of them not being able to make up their mind, fence riders say what's needed/wanted to be heard.