Of course, there is a Mitzva of Hochei’ach Tochi’ach. However, the Torah cautions, Lo SIsa Alav Cheit. Correcting your friend must be done in a way which does not offend him, and which will cause him to do the right thing.
The first step is to determine whether in fact the “Halacha” in question is a bona fide Halacha. For this, do not rely on “what you were always told,” or even what your Rebbe said in High School. Go ask a competent, practicing Rav. Proceed only if a true Halachic issue is present.
The second step is to determine whether there is some extraneous factor which will prevent the other person from listening to you. (For example, is there some existing relationship between you and him in context of which he will automatically feel defensive when you approach him?) If so, get someone else to approach him.
The third step, once the first two have been satisfied, is to prepare a nonthreatening, friendly, SINCERE, way of presenting the problem to him. (It is important, for example, that this is done in a dispassionate manner, without condescension.)
My experience (repeated many dozens of times without fail) is that if this procedure is followed, people are grateful for the assistance and happy to change.