If a person which mokes it a habit to commit a few sins wants to abandon one of them, one should not be said to him, "If you do not quit other sins, keep committing this sin too". The less the number of sins committed, the better. It is a sign of iman to abandon a sin out of fear of Allah. "A Muslim who remembers Allah or who fears Him once in his lifetime will get out of Hell" (Tirmidhi).
If a sinner holds the fast and gives the alms, we should say to him, "Do not abandon practicing them". If he does not does these acts of worship, he may become altogether estranged from the religion. "Curse be upon those who cause people to despair of Allah's mercy and to hate the religion. Make things easy; do not make them difficult" (Bukhari).