Hi on my buddies 97 328is we had to disconnect the battery because the radiator fan or the engine fan will not turn off even when the engine is completely turned off. Also the temperature is fine, not overheating?

There's a fault in the control circuit for the aux fan, (bad switch, shorted relay, etc).

how much about will this be to fix? what parts exactly will need to be replaced in a worst case scenario?

The answer to both of those questions is determined by what's wrong. And that won't be known until the fault is identified.

Did your friend ever solve his problem?