Am I sinful for not denouncing my friend for wearing shorts that reveal part of his thigh, even though I avoid looking at it? And is it obligatory for me to advise him or to denounce him?

It was previously favored that a man's thigh is considered 'awrah by the majority of scholars. Objecting to its exposure falls under matters of disagreement where no objection should be raised. It is well-known among scholars that "there is no objection in matters of jurisprudential disagreement (ijtihadi اختلاف) for which there is no clear text or consensus." However, it is recommended to advise him to cover his thigh to avoid disagreement, because scholars "are unanimous in encouraging avoiding disagreement if it does not lead to neglecting a Sunnah or falling into another disagreement."