The key to lose weight is by eating right and staying physically active. This balance is the only key to stay healthy forever.
Now your friends must be blessed with very good metabolism
because of which they are able to eat a lot and yet not get fat.
You may also achieve good metabolism by staying physically active, this will help burn the extra calories which are stored as fat, provided you eat carefully and only healthy. Else your body will be busy burning the extra calories that you consume now and will not be able to burn the one's which are stored overtime.
Wishing you good health.