Set Workflow to a specific cell/row not the column



I'm trying to set up an automatic workflow that will trigger an email alert to a person when a particular cell has been populated but can only seem to be able to do it for a column and not a specific row.

Example is: Person A is assigned a client. When Person A completes Task 1, Person B is to Peer Review that completed task.

I would like an email sent to Person B when Person A marks Task 1 complete. There are 4 people on this team and each has been assigned to peer review another team members work before I do the final review. I have the workflow set up to send me and and my #2 an email when the peer review is done, but that's easy bc we are the only ones to do the final review. The peer review notifications are seeming to be a little more difficult since the rows are assigned to different people.

Is there a way to set up the workflow for a row vice a column?