This achievement is missable. You cannot get this achievement until after you have convinced Nericon to let you cross Mardanues Plaza, see the Oaken Resolve achievement for more information.
First you need to trigger a cut-scene with Tom in Crow's Nest, then once you have rescued Anne from the hands of Luxurius, you can visit the two in their house on Lanilor Lane. Make a save then read the minds of both. By choosing the new conversation options you can persuade Anne to leave Tom, who is definitely not the man her father would have liked her to marry. When you have the achievement, reload the save so that no XP is wasted.