No, but the original is not correct either. All of those things - a walker who was trained and who is a friend - describe Mercy, not Mercy's world. It would actually be improved by leaving the comma, but changing "Mercy's world" to "Mercy lives in a world that"
Thank you! I see your point about the first sentence. Can you tell me what rule would the sentence followed under? I mean the one where you leave the comma in. I need to study it.