3: I have been in a similar situation before 2 years ago and I think that the best thing to do is to try to explain to him that he is too young to buy a cellphone. (FULL STOP) He can wait for one or two years to buy it. You can suggest him that he can use sometimes your cellphone if he wants to contact with his friends.
2: A similar thing once happened to my friend. (FULL STOP) Try to explain to him that spending a lot of hours on the Internet is unhealthy. He can try to do some sports or find a hobby which appeals to him.
4: As a teenager, I believe that the best thing to do is to persuade your friend that yesteday you were very busy and had a lot of stress. I can understand how your friend must feel but I think she absolve will forgive you.
5: I can understand how you must feel about your son. (FULL STOP) As a teenager I believe that the best thing to do is to explain to him how important it is to exercise, the benefits of being healthy and having a healthy body. Moreover, try to explain to him how important it is to be a good student for his future studies in university and moreover in his job.