Player A activates The Monarchs Stormforth, tribute summons a monster from hand by tributing 1 monster on player B's field and sends it to the graveyard.
Player B has Interplanetarypurplythorny Dragon in hand, can he special summon it in response?
If a monster you control is destroyed by battle or card effect and sent to the Graveyard: You can Special Summon this card from your hand.
Tributing a monster is not considered destruction, so the requirements to activate IPPT Dragon's effect have not been met. Note that any card effect is not considered to destroy anything unless it explicitly states so.