Yes you can give guinea pigs the occasional apple. This should not comprise a large amount of the diet as they are hind gut fermentors---their gastrointestinal tract is not structured to digest the sugars found in fruits. The diet should mainly consist of timothy hay and a pelleted guinea pig diet. Supplement with fresh green vegetables. You will also need to give oral Vitamin C tablets. Water should be available at all times. Feed treats sparingly.
Yes you can give guinea pigs apple as an occasional treat but NEVER give them the pips as these are poisonous for them