my nieces car is having charging issues, shes tells me she had the battery and alternator replaced and it is still causing her problems . what could be causing this

First question is how old are the battery cables? If they are more than 5 years old, I would change them anyway. They can fail internally, and good 1 gauge cables are not that expensive. After that, I would have the charging system checked in the car. There could well be a faulty voltage regulator.

@tennisshoes - there was NEVER any ECM controlled alternator in any 198ties and 199ties Buick