Your analogy doesn't really work that well.

You know greenhouse gases are a problem, and you're just an average person. Fixing the world is not your personal problem to solve - but you can minimise your impact where you can, walking more or switching off lights or buying local produce to voting for people into power who aren't denying the problem.

OP can't solve the entirety of the modelling industry's bodyshaming practices or fix the girl's self esteem, but generally speaking if you're going to tell someone that their individual interaction with someone else makes them a jerk, you can at least offer an idea of what would make them not a jerk in your estimation.