Is it obligatory to tell a brother who does not pray about a dream of a snake coming out of his mouth, and is it a true vision or from Satan, and how should he be dealt with, especially since he claims repentance but does not attend prayers?

It is not advisable to tell someone about a disliked dream, lest they interpret it in a way they dislike. The Messenger of Allah (peace and blessings be upon him) said: "A good dream is from Allah, and a bad dream is from Satan. So if one of you sees a dream that he dislikes, let him spit to his left and seek refuge with Allah from Satan; it will not harm him, and he should not tell anyone about it." However, if someone is negligent in prayer or persistent in sin, then they must be advised, enjoined from evil, and effort should be made to help them abandon it.