If you are lonely, can you have interest in gambling simply because of your loneliness?
It basically depends on your personal interests and what you stumble upon when you are sad and stressed out and looking for a way to kill time and prevent yourself from drowning in depression that might be more harmful. So, someone who might had some interest in gambling in the past and has become lonely just recently might turn back to it.
How would you judge someone who says they don't gamble because it's fun, is that a sign of a gambling addiction?
Well, a lot of people don't gamble just because they find it fun, some might do it because of loneliness and depression and others might be doing it for earning money.
If you are making $600 a day, that's $20000 in a month, and because you are only you start having interest to gamble, what do you think this is?
It's either addiction or reckless behavior just because one has a lot of money and they can afford to lose that much in a month. Whatever the reason is, it shouldn't be done.
Later he open up to have been using $500 for gambling per day, he said all these started when his wife left him, that he isn't into gambling at all, but I can't ask why his wife left him in the first place, but does loneliness make people become addicted to gambling or it's something else.
He needs help, obviously. He is going into depression and he is definitely looking for ways that can save him from that and he is trying to do that with gambling. If he is a close one, try to help them out, spend some time with them, and try to cheer them up, also try and get them to do some other activity instead of gambling and wasting that much money. Video games can be a good way to convert your mind from such things.