Canadian Passport -US GreenCard

[Steven]

NEXUS card won't help you, at the interview they will ask what your legal status is in Canada and the US (in fact it asks on the application form) and add that in to the NEXUS database, they will actually want to see your LPR card and add that date in and you have to notify the NEXUS office every time you renew it. NEXUS is a replacement for a passport, it confers no other benefits at all.

Can't comment on your colleague, I assume he claims to be resident in the US, pays taxes and has an address there. From what you've said though he doesn't mention his LPR status, just goes in as a visitor which is misrepresentation and illegal, I'm surprised CBP hasn't picked up on it yet frankly.

However generally speaking the rule is that if you spend more than a year outside the US, you lose your LPR status. I've actually sat in secondary inspection at Sweetgrass and watched CBP seize the LPR cards of a Chinese couple who had spent several years in Canada.

If you lose it, it will be a hassle to get it again. USCIS really don't like applications to sponsor people again, you can file I-407 and surrender it then re-apply later on I suppose, but talk to USCIS before you do it.