Is my guinea pig happy alone?

She is alone in a 30 by 18 inch cage. i am at school from 7:00 to 3:15 i am not home ALOT during the weekends but mostly am. She seems happy when i am home and is jumping and running around. i have alot of toys for her and she plays on the floor alot. i give her fresh kale every day and hay every other day. Is she happy or should i get her a guinea pig friend? if i should how do i convince my parents to let me get one TODAY! since we are going to the pet store today!!