There is no doubt that this is blameworthy. Losing something after once having it is something to seek refuge from. Going back after being from those who pursued doing good deeds is something criticized in the Sacred Texts. In any case, whoever wants to observe a Sunnah then he has done good. The person in question sufficing with doing the bare minimum integrals of Islaam does not mean that he has done everything that Allaah has asked from him. This is because there are other obligatory actions beyond the integral aspects of Islaam. Therefore, he must perform everything Allaah has asked of him. In a narration it is mentioned,
“My slave does not draw close to me with something more beloved to me than what I have made compulsory for him, and my slave continues to draw close to me by voluntary deeds until I love him…” (Bukhaary, 2502)
Hence, if he leaves out voluntary worship then there is no consequence, and his actions of obedience which he observed before becoming lax will be rewarded. Those deeds are registered in his register of good deeds, if Allaah wills, and these deeds cannot be rendered null except by apostasy, and we seek Refuge in Allaah from that. It is upon brothers, friends and colleagues of a person who used to be active in doing good deeds and then became lax, to remind him of what will make him closer to Allaah. They should help him return to being how he was, and he should seek help to to return to being how he was. Allaah, Most High and Exalted, says,
(وَٱصۡبِرۡ نَفۡسَكَ مَعَ ٱلَّذِينَ يَدۡعُونَ رَبَّهُم بِٱلۡغَدَوٰةِ وَٱلۡعَشِيِّ)
“… and keep yourself patient by being with those who call upon their Lord in the morning and the evening…” (Al-Kahf, 28)
The negligent should not be left without reminders; rather they should be helped.