Strange question!!

My brother in law was told by his dentist to stop drinking orange juice. I thought that was weird but he has been brushing really hard with his toothbrush and he has a cavity or two each dental visit. I can only think that maybe the acidity of the juice acts with the ph of saliva or something and contributes to his high risk of caries. If anyone has any ideas of why this is please tell me so I can tell him!! Thanks