I honestly believe my best friend is too hot for her boyfriend.
Why do you think that your best friend is too hot for her boyfriend?
She tells me that she is too hot for him all of the time and I totally agree.
By agreeing with your friend that her appearance is too hot for your boyfriend, you are validating her shallow mentality and this is wrong. Instead, you should remind her that it is not only physical attraction that makes a person hot.
I think she's too hot for him anyway, and I'll tell her regardless of what you say.
You can say she is too hot if you choose to, but you will only be playing into her immaturity about her body and looks. Don't support superficiality in your friend. Show her what her true value is.