She found out about his hobby so what? We, men, don't need them to be supportive of what we are doing, as long as we don't cross the line. That's where they come in. I have been gambling for years too and my wife doesn't actually care what I gamble as long as I am responsible, can supply them with their needs and a little luxury. Now that's trust.
Maybe she is just exaggerating because of his previous experience, like a trauma, but it can be fixed thru explanation. They won't go farther if your cousin stops and won't be happy anymore. Worse, he will hide the hobby and it leads to an unhappy relationship filled with fights.