The discard is a cost, the monster doesn't get special summoned until resolution of the effect, after cost is paid. If you interrupt the effect after the discard the effect doesn't go through. So chaining fiendish after cost is paid = spell card lost and no special. Easiest way to know the cost, or requirements of a card needed for activation, look for the semicolon. Not every card that has a cost will have one, but if you see one, it is often displayed after a cost, or requirement. For others, who you come across, who have no idea what they are doing, just explain to them what a cost is. That usually does the trick for me when the situation pops up.
To clarify for this situation, timing is a thing, but not once he states he is activating the effect of summoner monk. Once he states that, he has to pay the cost, or at least that is how it is suppose to play out. Obviously any other point prior could leave you to interrupt the card before hand, like switching it to defense or using chain during another game state, that didn't have his cost come into play. For the sake of not typing a wall though, yes, if he states the effect activation, he needs to pay the cost.
