Actually this could back fire rather badly. If you flat out tell a woman they are fat, then it could send them into a depression (pending how dramatic they are) and cause her to eat more, comfort food.
Personally if you want to help her lose weight, then encourage her to do so. If you talk to her on the phone when you're away be intrested to see if she has been eating healthy and excercising properly, but be nice about it. Encourage her when you see her like if she is sticking to the diet, tell her she looks pretty and she's lost some weight, that will help motivate her to lose more. Some times a simple compliment is enough. Just don't go extreme with it, compliment little by little so she feels like she's actually getting some where with the diet. Either way just keep supporting her the best you can and everything should work out fine =)