Is offering condolences to the family of the deceased part of the religion or part of customs? And is a person rewarded for going to offer condolences to a friend who has lost a loved one?

Consoling the afflicted is a religiously established matter, confirmed by the Prophet, peace and blessings be upon him, through both word and deed. Whoever consoles his brother in affliction will be clothed by Allah in garments of honor on the Day of Resurrection. The Prophet, peace and blessings be upon him, consoled his daughter upon the death of her son by saying: "Indeed, to Allah belongs what He took and to Him belongs what He gave, and everything with Him is for an appointed term. So, let her be patient and anticipate the reward from Allah." Consolation is among the acts of devotion through which a believer earns great reward if he purifies his intention and seeks the reward from Allah.