Im an owner of a plumbing company and i have seen this. It was the weirdest site to see water coming from the roof vent. It was caused by a clogged sink p trap, and drain, and when the washer drained the force of pressure shot it out the roof vent.
where the drain and water stub thru. place a card board box around it and concrete around the box. when it dries remove extra cardboard and fill inside with sand