Actually, I would like to know this too... I had sent Greirat to Irithyll but did not defeat Pontiff... Instead what I did, was skip into Lothric Castle early, so once intside the Castle...maybe then this triggered Greirat death in the Irithyll sewer? Yes, Siegward was at the kitchen, so he should been able to rescue him anyways. So I do have to assume by skipping into Lothric Castle while Greirat gone to Irithyll then this triggers early death.