There will likely be a way to retrieve the codes without a scan tool... however you will need a service manual to learn how to force the Engine control module into diagnostic mode... I dont' think a haynes manual will have that information in it.
My suggestion is go to your nearest dealership and see if they will sell you a manual for your wife's vehicle...(won't be cheap)
Chance are unless you are a technician they may not sell it to you. It is definitely worth a shot though.