Of course, if he looks like he’s not just going to visit his sister - if the border guard even suspects that he’s planning a complete move (“Why are you taking a dresser and sofa for a 5-month visit?”) then they may deny him entry. Things like what job he has, where he will stay and what he’s going to do, how much money, etc. may set off warning flags. If he seems evasive or his story is inconsistent… depends how alert or ornery the border guard is.
The US border patrol are little dictators and can do whatever they want. They can ask questions that an employer or others cannot (“Have you ever been arrested?”) They can search watever they want; if you object or complain it just annoys them. Their attitude seems to have gotten worse thanks to Cheney’s war on terror. They can do whatever they want, and you can’t “demand to speak to their boss”. If they deny you entry, you’re SOL until you do a whole bunch of paperwork with the embassy in Ottawa.
One guy who was sent on a business trip from our company got turned back at the airport because he was drunk and uncooperative. They based it on the fact that he had a conviction as a juvenile when he was 17 (he was in his 50’s) and “we think you resemble one of our wanted felons”. Big embarassment, waste of thousands of dollars.
So - crossing the border under false pretenses - risky. Working illegally - very risky. If he’s caught, as others point out, that’s it for visting the USA for quite a while; no vacations to Vegas or Disneyworld, no Florida suntanning. You may not even be able to pass through the US on your way elsewhere. (Vancouver to Australia refuels in Hawaii).
As for getting a green card - no education, no in demand skills, … Being white and english-speaking does not count for much. Basically he’s in the same line as anyone from Mexico or points south who has a seester in the USA. It’s a pretty long line.
He’s probably better off trying to do the 6-month “work your way around” visas that places like New Zealand (and Australia?) offer to the unattached younger tourists. But - the demand for untrained workers is pretty low compared to huge supply in the USA.