Dog wont eat help!!!!!!?

About 2 weeks ago we adopted a saint bernard. The man who surrenderd her to us said she eats anything and he just fed her whatever was on sale. We got her pedegree large breed food and she ate it fine for the first week. Now she refuses to eat it at all unless i mix some broth or something into it and then she usually just licks it. I am getting worried because she is already too thin because the guy only fed her once a day before and i am trying to get her to gain a few pounds so she will be at a healthy weight. My boyfriend is always giving her table scraps which i tell him not to but he is the most stubborn man on earth and will not listen to a word i say! is it because the people food? What can i do to get her to eat her food? I have been feeding her 2 1/2 cups twice a day and at first she would eat it all in one sitting...now it just sits there all day...i am so worried about her what can i do? Also what else can i give her to make her gain weight she needs to put on another 10 to 15 pounds, i just hate seeing her look so skinny, the vet says there is nothing wrong with her and she was dewormed last month and hasnt had a flea on her so i dont know what the problem could be!