Human relations are very precious. It is up to our mental approach whether we give worth to friendship or not. If someone doesn’t want you two to sit together, it may be his or her psychological problem. Feel the gravity of the situation and avoid sitting besides while the third person is there. If any of our action hurts one, we should avoid as much as we can. Try to make her friend of yours. Invite her in games you play. When you both want to talk together try to share with her. It is difficult but it would be a great task from your side.