Some people can't help talking loudly in the library. If it doesn't affect me, I won't mind my own business, because more is better than less, and we are all students, so we may not be able to control our temper. Young people are prone to some quarrels and contradictions. If the noise is too loud and disturbs many people, you should come forward to remind him at this time. You can do some quiet actions with him, such as putting your hand in front of your mouth to let him know what you mean.