1. You've only "heard" that from lying Apple-hating trolls, not reputable Apple users or representatives.
Of course it's not true -- every consumer-grade computer made today is hackable given the proper conditions. It's certainly much more difficult to hack into an Apple computer than most others (Unix is very tough, which is why Max OS-X, based on Unix, is also tough), Linux is pretty tough, even Windows is much tougher than it used to be. But, for instance, given physical access to the computer, a hacker can hack into any machine given time and knowledge of the operating system and its vulnerabilities.
Stop listening to the lies of the Apple-hating trolls. They aren't doing you any favors. And if you have any Apple-loving acquaintances who are so naive that they believe such a tale, you can tell them that Dayglored, happy user of Macs since 1984 (that's 30+ years), says "That's not true".
2. It's extremely unlikely that the Mac has actually been compromised by the pop-up or your friend calling the 800 number. But it won't hurt to find a reputable free anti-malware scanner for OS-X and run it on the machine. There's a decent chance that this isn't the first time your friend has made this error and done something potentially harmful to the computer.
3. Tell your friend to get an ad blocker (e.g. AdBlock) for the browser, and use it.
Best FRegards,
Dayglored
You can also tell her that Swordmaker, the keeper of the 700 member Mac/Apple ping list on FreeRepublic, and a Mac user for longer than dayglored, will also tell you the same thing. Sit me down in front of any computer and I can get into in pretty short order. . . no matter how strong they think their protection might be. Physical possession of the computer basically means it can be owned by a hacker. It is REMOTE hacking that is very difficult and usually requires the cooperation of the user, and getting in by malware attack that is even harder on a Mac.