Is it permissible to spy on a son's wife who leaves the house frequently, by sending a trusted person to find out her destination, knowing that she prays and fasts, and that the husband is acting strangely, which is said to be due to sorcery?

The default is to assume the best of a Muslim, as the Messenger of Allah, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the falsest of speech," and Allah Almighty said: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy."

However, if there is a strong suspicion of an apparent sin, then it is permissible to spy to prevent it, as stated in Tuhfat al-Muhtaj fi Sharh al-Minhaj and Al-Ahkam al-Sultaniyyah by Al-Mawardi.

The husband must protect his wife and keep her away from doubtful situations, otherwise he falls into the legally reprehensible act of dayyoothah (cuckoldry/lack of protective jealousy).