nobody wants to think about this, especially when their money may be at risk, but the fact of the matter is no programming, no encryption, is unhackable. heckfire, government agencies and military spend beaucoup taxpayer bucks trying to protect "their" information from compromise and fail. any program that can be built can be unbuilt, not to mention intentional "backdoors" built into the thing to allow authorized folks in for maintenance and repair.
i do not know much about this but my son does. he does troubleshooting for wells fargo's programs, offline and online. he first tipped me off, as a fellow online poker player, about the fact that there is no such thing as TRULY RANDOMIZED R.N.G. (random number generating) programs that the entire online gaming industry relies upon. patterns develop in the program and you may have seen some pretty weird stuff as a result, like time after time you flop the nuts and the other guy hits exactly what he needs on the river to win...not so weird if it happens once in a while, but when it happens five times in a row you gotta scratch your head and wonder...or if you get multiple pocket pairs in a row, my top count so far is NINE pocket pairs in succession.
as far as some other guy seeing your hole cards, my son tells me this is possible, probably through the aforementioned backdoors...not a very comforting thought. he also advised me to be more cautious with my money playing online than i would normally be playing at a brick and mortar casino, and to remember that people always figure out a way to cheat at cards, online or offline.
he also said that if anything looks suspicious to ALWAYS report it to the casino...taking a screenshot was a great idea to provide the casino with the info they need to track him down. he also said that the casino, like any business, is going to want to protect their reputation, so they are not going to admit that anything like this is possible, but that they will pursue and likely prosecute any shenanigans vigorously.
look around the net a little and you will see that security issues are real and that the online gaming industry is starting to talk about it a little...as far as truly compromising a gaming site, have a look here:
How We Learned to Cheat at Online Poker: A Study in Software Security [Cigital]