Originally Posted by
GRALISTAIR
Extremely unlikely you will be refused entry. You may get a few extrat questions. Hell, I am a Brit - with a Green Card and I get bombarded with questions. Go with the flow. You will be OK.
You have a green card and so are not subject to the rules of a vwp citizen who does not have a green card. Indeed, as a green card holder you don't even have to present a passport to the CBP. As 747 has explained, he will not be fine, and worse, he risks a ban from the USA which will make it harder for him to get a green card should he and his gf later marry and desire to live in the USA as a couple.
Originally Posted by
B747-437B
Yes, it could make a significant difference. You will now be clearly entitled to use the VWP (technically, the VWP requires you to have a "return ticket" but that interpretation is often loosely applied to include separate tickets) without relying on oversight. You will also be demonstrate temporary residence in Mexico and intent to return to your country of citizenship, thus creating a tie to the UK that was not hitherto in place. Taken together, these create a far lower risk scenario than otherwise.
Remember, the main thing is to not attract attention to begin with. Once they start poking around in secondary, it is a toss up whether the officer woke up on the right side of the bed that morning.
Isn't it the case that to comply with the rules he has to return to the UK and not just buy and cancel the ticket?