Link please.
He/she must be stopped from spreading misinformation.
Normally you add a diode across an inductive load (solenoid/fan/etc.), but in this case (comuter fan) I don't think it's needed, because of the electronics inside the fan. No harm done if you still add one.
The missing base current limiting resistor is a problem. That will burn out the Arduino pin.
1k should be ok for a TIP120 and a ~300mA fan.
The SSR I see already has a resistor built-in, so don't add another one.
Leo..