Sentinel&Polearm Master: X move through an ally’s space the speed of X becomes 0. Where should X position itself?

Sentinel & polearm master . X move through an ally's space (inside the ally's space, it also enters inside an enemy area "reach 10ft" and X receives an OA).The speed of X becomes 0. Where should X position itself? in the Square before the OA? X chooses another legal adjacent?

— Rober (@Rober_lpgc) May 7, 2020

As a DM I'd have them get pushed back to the unoccupied space they left to enter their ally's space.

— Dan Dillon (@Dan_Dillon_1) May 8, 2020

I prefer "the creatures explode."

— Dan Dillon (@Dan_Dillon_1) May 8, 2020