What if they were given one of the sets of two seats on either side of the plane, should they still have to buy the thrid seat or just have to deal with each other? That way they are only invading one anothers space or the aisle.

Does Southwest allow couples of size traveling together to only purchase one seat or do they force each person to purchase an extra seat?

Did the OP's friends consider buying two Bizclass seats? It might have worked out to about the same after buying a third ticket.

Just some thoughts