You're right to be worried about your friend Jolin.

你对你朋友 Jolin 的担心是没错的。

I'm worried about my friend Jolin. She wants to be a model and she's thin, but she thinks she's fat. She's always on a diet.

我很担心我的朋友 Jolin。她身材苗条，想成为一个模特，但她觉得自己很胖，总是在减肥。