Can someone with brain damage from substance abuse have a normal eeg?

Yes, possibly: Eeg can be normal even when brain abnormalities actually exist. Neurologists would be assessing a wide variety of findings, not eeg alone. If the person was actively having withdrawal seizures from certain drugs though, the eeg would show seizure activity at that time.

Yes it is possible: The important issue is not what the eeg shows but to be sure that the substance abuse is under control and that the person is getting good rehabilitation treatment.