Yeva, Nature's Herald; Primordial Hydra, Corpsejack Menace

[prayos]

So, I was playing my cousin in a casual game the other day. He built this deck with this idea in mind, and I let it go, cause I wasn't 100% sure of the interaction. Here's the scenario. He has {Primordial Hydra} and {Yeva, Nature's Herald} on the board. During his upkeep, the Hydra's ability triggers to double the counters. In response, he flash's in {Corpsejack Menace} thanks to Yeva. My question is does Corpsejack still double the counters? I want to say no, because the trigger was on the stack before Corpsejack was out. Thanks.