Hi,
What year is your friends Mustang?
The radiator fan should come on at a designated temperature. If that OBD1 scanner activates the fan, that mean the Relay side of the fan is working. I would look at the ECT sensor next. If your 100% sure the fan does not come on when the engine warms up, you need to find out why the control side isnt activating it. You will need to probably look at some wiring diagrams and go from there.
As a last resort, you could pun the fan on a switch if that would be easier for you to do.
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1993 2.3 Mustang Convertible 
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