Q - If a devotee commits some sinful activity at some stage, and later he resumes the spiritual activities like chanting of Hare Krishna Mahamantra. How to behave with him?
Spiritually, this world is a prison house for all those who do not believe in God, do not serve Him, or do not follow His instructions. That is why all are not at peace here. This unrest is sort of punishment from Supreme Personality of Godhead.
Supreme Lord Himself or His personal Associates descends from times to time to inform all living beings about the correct path and their real duty. Those who agree, are released from this prison.
Supreme Lord Sri Krishna mentions in Sreemad Bhagvad Geeta (18.62)...
tamev sharanam gachh sarvabhaven bharat,
tatprasadarparam shantim sthanam prapasyasi shasvatam.
When a child begins to walk, sometime he fell and get up. When a student is studying in school / college or a person is doing business or one is maintaining his body or is managing family,......while doing that, up and downs keep on happening,
During bad times, if one leave or stop doing that activity, then he/she is not intelligent.
We are practitioner of Devotional path. Likewise another devotee is also practitioner of same path. A practitioner has affects on his/her life of up and downs due to previous samskaras. This is natural.
A sannyasis/ grihasthi, women/men, child/elderly, who follow devotional services and are also engaged in opposite services (averse to devotional services), for those people Founder-Acharya of Sree Chaitanya Gaudiya Math, Srila Bhakti Siddhant Saraswati Goswami Thakur Prabhupad's one of the dearest disciple, Srila Bhakti Dayita Madhav Goswami Maharaj ji, has clearly mentioned that we must respect them, but do not associate with them.
Shrila Bhakti Vicar Vishnu Maharaj, Sri Caitanya Gaudiya Math