I can't predict exactly what will happen, of course, but I strongly suspect that by this time next year your cousin will have lost all of his winnings (or even more). Even if he's been successful in the past, there's no guarantee he'll be successful in the future. Games like this are based solely on chance - and the odds are always stacked against you.
Nowhere in the Bible does God promise to bless us when we gamble (even if we pray for success, as some people do). One reason, I believe, is because gambling is almost always motivated by greed, and the desire to get something for nothing (or at least for very little). But greed is wrong - and it's not only wrong, but it's also a dangerous sin. Jesus said, "Watch out! Be on your guard against all kinds of greed" (Luke 12:15).
Why is greed so dangerous? One reason is because we easily become consumed by a desire for things and for money. Greed also blinds us to the needs of others, and makes us selfish and covetous. Most of all, it can cut us off from God. The Bible is right: "The love of money is a root of all kinds of evil" (1 Timothy 6:10).
Pray for your cousin, and urge him to reexamine his goals in life. Most of all, urge him to put Christ at the center of his life, and not be led astray by greed or any other sin.