How do you tell your friend she's fat?

So my two friends (one of them is 4"10 and a hundred pounds and the other one is 4"8) they were not he see-saw and the one that was 4"10 was way up in the air and 4"8 was on the ground. She's also a whole lot younger than 4"10 (by like 6 months). So I guesstimated that she was 110 pounds and did her bmi which she was 24.3!! What do I say to her!!!!