Tafsir of Al-Kahf 18:42

{And his fruits were encompassed [by ruin]} This is an expression for his destruction. Its origin is from the enemy "encompassing" someone, because when they encompass him, they have mastered and seized him. It was then used for every form of destruction. From this is the Almighty’s saying: {...unless you are encompassed [by enemies]} (12:66). Similar to this is their saying: "It came upon him" (ata 'alayhi) when it destroyed him, derived from "the enemy came upon them" (ata 'alayhim), meaning they came upon them while having the upper hand.

{And he began to wring his hands} The turning of the palms is a metonym for regret and sorrow, because the regretful person turns his palms from back to front. Just as this is also expressed by "biting the hands" or "falling into the hand" (regret). Because it carries the meaning of regret, it is transitive with 'ala (upon), as if it were said: "He began to regret {what he had spent on it}," meaning what he had spent on its cultivation.

{While it was fallen down upon its trellises} Meaning that its trellised vines had their trellises collapse to the ground, and the vines fell upon them. It is said that Allah sent a fire upon it, and it consumed it.

{Saying: "Oh, woe to me! I wish I had not..."} He remembered his brother’s admonition and realized that he had been brought to ruin because of his polytheism and tyranny. So, he wished he had not been a polytheist, so that Allah would not have destroyed his garden. It is also possible that this is a repentance from polytheism, regret for what he had done, and an entry into faith.

It was read as wa-lam yakun (and he was not) with both the ya and the ta. The verb yansurunahu (they help him) is construed according to the meaning rather than the literal wording, similar to the saying: {...a party fighting in the cause of Allah and another disbelieving, seeing them [as twice their number]} (3:13).

If you ask: What is the meaning of his saying: {...they help him other than Allah}? I say: Its meaning is: "Are they capable of helping him other than Allah?" That is, He alone is capable of helping him; no one else is capable of helping him, except that He did not help him due to a deterrent, which is his deserving to be forsaken.

{And he was not a defender} He was not able to protect himself by his own power against the vengeance of Allah.

{There, the authority is with Allah, the Truth. He is best for reward and best for the outcome.}