be always nice to her, don't try to grab her boobs after you finish her quest, and don't use the dialogue on why she's lonely, etc. Only use the first two dialogues when at dinner. After that leave and come back, you'll follow her to see her son, then leave again, and come back, talk to her, admit you followed her, and be nice, never pushing, she will be drunk and will have the first scene with you. Then just leave for a while and come back for the last scene.
The secret is being nice, and wait until she makes her move on you.