What should I do about seeing someone I know who is married committing adultery with a woman, especially since this happened in one of my relatives' homes?

Adultery is a major crime and a great indecency, and its perpetrator is exposed to punishment in this world and the Hereafter if he does not repent. You must advise the two adulterers and warn them of Allah's wrath, and threaten them with informing those concerned if they do not stop. You must conceal their sin as long as they do not openly commit the disobedience, for whoever conceals a Muslim, Allah will conceal him.