well that's a whole different story. Many online gamblers are skeptical about the places they play, and of course they should be. As long as he plays at reputable places, he should be okay I think.
The idea behind online poker is the house doesn't want to cheat you. In poker, the casino/poker room makes money from rakes, which is a % taken from the pot every hand no matter what. Doesn't matter who wins money, who loses money, the house gets paid via the rake.
Other games are a different story. Blackjack and slots are the person vs the house, so in this case the house does have a strong interest in what happens each hand/spin. Partypoker implemented Blackjack into their software, and judging from my personal blackjack experience, the partypoker one seems to be rigged. Plenty of people will tell me that Im just mad that I lost money, which is true, but I really really dont trust the blackjack there. In fact, a few of my favorite poker rooms have started offering blackjack too, and I was so disgusted with my experience at PP that I refuse to play Blackjack or any other table game online.
This is not meant to scare you or your friend away. If he is winning then more power to him! The biggest problem is knowing when to stop. In real life I usually go by the "3 losing hands in a row, go to a different table" strategy when it comes to blackjack, and it works very well. Also, knowing when to get out ahead is important too. He should set goals such as "If I make $100, I'll stop playing for <determined period of time>" This helps prevent stupid losses. For instance, if you go to a casino, or online with $20. You luckily turn this $20 into $80.. well now you feel great, and you treat $60+ as winnings.. meaning, you bet more, you play looser. Well the more you bet the more you can lose, and pretty soon after a few bad cards you can be back at $0, when you could have taken a break at +$60.
HTH