People above have told you how missed triggers are handled in tournaments; the official documents that they quoted are meant to be used by judges in sanctionned events. In a casual playgroup, missed triggers are usually more of a gray area; you guys have to decide how you wish to go about it. The tournament documents can be used as a reference if you wish, but they don't have to be.Quote from TheDesacrator »Can a player miss a trigger that isn't a may effect? Happened today in an edh game. Player attacked me, his creature said whenever deals combat damage to a player X happens. He didn't see it so I just continued playing. Do I have to call my opponents triggers if they aren't may triggers?
That said, I don't think I've seen a group call someone a cheater for not reminding an opponent of their trigger. The philosophy applied in tournaments that one is responsible for one's own triggers is sound and pretty natural for most people.
Personaly, in casual games, I always remind my opponents of their mandatory triggers when I see them in reasonable time, because as I see it, from a strict game rules standpoint, those triggers should happen; but that's just my own ethics. As for them remembering their trigger late, if it's a mandatory one, I would follow the philosophy of the Judging at Regular (J.A.R.) document and make it happen if it's remembered within a turn cycle and it's not disruptive in relation to what happened afterwards in the game. For forgotten "may" triggers, I would be less lenient; in my mind, all opponents would need to agree to be good sports and let it happen, otherwise it's missed.