What is the punishment for not fasting without a reason?
It is forbidden for a Muslim not to fast without a proper reason like illness or travelling. This kind of act means a disobedience to Allah.
One who does not fast without a reason cannot compensate for this by fasting any other days. According to the 184th and 185th verses of chapter Al-Baqarah, only travelers and ill people are allowed to postpone their fasting duties.
This person should repent, ask Allah for forgiveness and abandon this behavior.