Are you feeding him the same food your friend was?

Could you take him over to visit the person who gave him to you? If he acts normal while there you'll know he's just going through an adjustment period at your house.

If he acts lethargic and won't eat or drink while in their familiar surroundings, then it's time for a trip to the vet.

In the meanwhile it wouldn't hurt to see if you could get him to drink a little gatorade or even lightly sugared water to keep him hydrated.