During the "Kobin's Mansion" level in the single player campaign, you can see the corpse of Archer from the multiplayer campaign. In the second room you enter (with the chandelier), kill all the guards. Walk down the stairs to the chandelier. There are some boxes directly under the stairs and a slightly opened body bag leaning against them. If you look inside the body bag you will see Archer. There is another body bag in the center of the room, but this one is completely closed. This is assumed to be the dead body of Kestrel.