What would you do with an aggressive dog with a bite history ?

My neighbour who is a senior citizen asked for my advice but I am not sure what to say.Her dog Aggie is a four year old spayed lab mix. Aggie has a bit history.Last weeK Aggie bit a dinner guest when she recached down to get her napkin. The dog has bitten my neighbour (as well as her daugter Donna who just moved in). Donna got very defensive of Aggie but MS Dennis (the older lady) is not to happy about this matter.Donna is only interested in protection training Donna and her friend where in the car Donna was driving Aggie was in the middle of them The friend casually leaned back and put her arm on the back seat Aggie bit the friend badly . Do you think Donna is correct beacuse she said that Aggie was only displaying appropriate protective behavior because she was trying to protect Donna. I think that Donna is completley incorrect because Aggie 's agreesion has nothing to do with protection Do you think that the dog is to hostile for any kind of training thanks for your help