1998 Ford Contour Catalytic efficiency problem/solution?

Engine Performance problem
1998 Ford Contour 6 cyl Front Wheel Drive Automatic 130k miles

Hello I have an odd question. I have a friend who I sold a car to, 98' ford contour v6 2.5l she has had the car for over a year and has been very happy with it. She always wanted a car with airbags. Anyhow its time for her to get the smog again but her check engine light comes on. The classic catalytic below efficiency problem. So as I understand it the downstream o2 sensor is the one that mesures the efficiency of the catalytic converter, so assuming that the o2 sensor is working is the only solution to replace the cats? Or can I just plug in some o2 sensors into the downstream connectors and let them read fresh air, would that trick the bcm/ or ecm? That is to say would it think that the cats were working, I only consider this because the car gets great emission reading and the only problem is the check engine light. They wont pass or even test a car with a check engine light on. My friend has no money and I feel bad because I sold her the car. Does anyone know if this is doable or would the computer realize the difference? Thanks for any input.