Im really worried about my cousin's weight?

I have a cousin who's 12 years old. Her name's Yasmin she weighs 150 pounds and is 5'3. I don't know what to do to help her lose weight. She looks very skinny though. She asked me for advice but I don't know what to tell her because i barely do exercise because I have a great metabolism , so I don't know what to say to her I'm really worried though please help. She tries to encourage herself to exercise but since she's lazy she always tells me she tries and that she keeps telling me that she says to herself "ok tomorrow i will start i promise" but then she never does anything. Plz help