The word "evidencer" may not be a word in common usage, but there are several synonyms that can be used to convey its meaning. Evidence-giver, proof-provider, testimony-giver, and demonstrator are some common synonyms that can be used in place of evidencer. If you are looking to describe someone who provides evidence or proof of a fact or argument, you might want to use one of these synonyms instead. Each one adds a slightly different nuance to the meaning, so consider the context and intended tone of your writing when selecting the best synonym to use.