A friend of mine told me that eating sunflower seeds would help me lose weight. Is this true?

Possibly. Sunflower seeds will provide protein and fiber which will fill you up. This may keep you from snacking on less healthy foods. Thereby, limiting your caloric intake. Good balanced nutrition and physical activity would are also worthwihile activities.
No. There is no food that will help someone lose weight. In order to lose weight a person needs to take in fewer calories and expend more calories, that is, eat less and get more physical activity. A proven way to lose weight is to cut down on portion sizes and empty calories like sodas and walk more. Walking 30-60 minutes a day 5 days a week is recommended. Work up to it gradually but stick with it.