My brother opened the bud and found that the wire inside wasn't connected properly and was loose.
@Tiera: I have edited your response in accordance with the Community policy on dangerous DIY and unorthodox repair advice. For clarity, please see the link below.
@terencepw: Some users have had success by charging the case, then placing the affected Bud in the case by itself. If the LED turns red, then green, please remove the Bud and repeat the process, with the goal being to see if the LED turns red, and stays red to indicate that it is charging. If you're successful, place the other Bud in the case, close the lid, then open it and check the Wearables app to see if the connection is recognised. I understand that this isn't ideal, and apologise for any inconvenience this may cause you.