"and claims they spent it all again online."

In which tax year? Did they live on some of the money (new appliance, better hotel room) and did they gamble some of it in the next tax year? You can't win in one year, regamble in that and the next year, and then expect to treat it as a loss for just the year you won it by amendment.

That's why you need proof, a log, a record. If they gamble with a member card, yes, that log is available.