Renewing B-2 Visa for Co-Habitating Partner

I am working in the US on a TN visa, and my girlfriend (Canadian citizen) entered with me on a six month B-2 visa as my co-habitating partner. She now has one month remaining, and will need to apply for a six month extension.

She has been in Trinidad for the holidays and will be returning to the States in the next few days. I know that she can submit an I-539 form with a $290 fee to apply for the B-2 extension, but is there any way she can have it extended on her way in through customs at the airport? If so, what documentation would she need? If not, would it be safe for her to tell the officer that she plans on staying longer than a month, but will be filing for an extension before that?

Of course, she should simply request a new B-2 right there at the airport.

She could tell the officer she plans to extend, this should not be a problem, since the regs allow for B2 to be extended for the life of your TN. Its just that right now they won't issue B2 for longer than six months.

She would need copy of your I-94, her old/current I-94, and proof that she still has residence in canada (remember, that is still a condition of partner B2).

Technically, each time she enters US she should ask for full six months.

I don;t advise I-539 however since i-539 for b2 will take 4-5 months anyways, so is a waste unless she is not planning to leave US for a while. best to get fresh B2 at re-entry at any good opportunity.

Thanks AGN, that was very informative!

One point I should clarify. My girlfriend has citizenship in both Canada and Trinidad. Although she does still have ties to Canada, she has stronger residential ties in Trinidad (parents, siblings, etc). If she's entering on a B-2 with a Canadian passport, does she have to prove ties specifically to Canada, or just more generally ties outside of the US?

Ties outside US.
Her Cdn passport entitles her to not need consular visa, but does not require that she live in Canada to get B2 status.

In other words, the same ties she used to originally get her B2 should still be in place.

Thank you both for your fantastic posts. Greatly appreciated.