He found a winning lottery ticket; is it permissible for him to cash it in?
The lottery is gambling, which is haraam. Allah, may He be exalted, says in His Book (interpretation of the meaning):
“O you who believe! Intoxicants (all kinds of alcoholic drinks), and gambling, and Al‑Ansaab (stone altars for sacrifices to idols etc), and Al‑Azlaam (arrows for seeking luck or decision) are an abomination of Shaytaan’s (Satan’s) handiwork. So avoid (strictly all) that (abomination) in order that you may be successful.
91. Shaytaan (Satan) wants only to excite enmity and hatred between you with intoxicants (alcoholic drinks) and gambling, and hinder you from the remembrance of Allaah and from As‑Salaah (the prayer). So, will you not then abstain?”
If a Muslim finds a lottery ticket on the ground, then he has not engaged in gambling, because he did not pay any money for this ticket, but this does not mean that taking the money is permissible, because he will be taking the money unlawfully.
What right does he have to take this money?
Taking money in this way comes under the general meaning of the verse in which Allah says (interpretation of the meaning):
“And eat up not one another’s property unjustly (in any illegal way, e.g. stealing, robbing, deceiving)”
A lottery ticket is not a permissible means of earning money.
And Allah knows best.