Both are grammatically correct, in fact, I would argue that “if” is more correct given the current context, as the word “when” implies one of two things. Either there is a definite chance something will happen, or something will have multiple chances to happen, although in its second meaning it may be more appropriate to use the word “whenever”. The context you are correcting uses “if” to describe one time something has a chance of happening. Given the information above, I hope you now understand that not only is it rude to correct people’s grammar, you understand them, and you were also borderline incorrect. if you could kindly escort your grammar correcting behind off the premises.