DoctorAllcome, the B-2 6-month visitor visa for common-law spouses/partners of US visa holders is for co-habiting partners (members of the visa-holder's household), and you have to provide proof of this at interview. The OP doesn't live with her partner. It also doesn't permit you to work, so she would be entirely financially dependent on him.
Well, I’m an American with immigrant family members and she absolutely CAN live here for 6mos on a tourist visa. It’s not “immigrant intent” if you have a return plane ticket and you not only intend to leave the US but you actually do leave after the 6mos. She would only break the law if she overstayed.
Yes, she would break the law if she overstayed. But US immigration aren't obliged to wait until that happens; they're perfectly able to make an informed decision about the potential risk of an overstay and deny the applicant a visa, or entry to the country.
Having a return ticket is not exhaustive proof of lack of immigrant intent (I don't think you can even get into the States, as a visitor, without a return flight or onward travel booked). Having provable residence and commitments overseas can help to demonstrate you don't have immigrant intent... but none of that guarantees anything. If they're worried you'll overstay or work without permission (and having a partner in the US is prime red flag for this) they can deny you entry at any point, and they don't have to explain, prove or justify anything.
The OP doesn't qualify for a B-2 cohabiting partner visa, and will almost certainly be declined for a standard 6-month B-2 visitor visa (as the 90-day entitlement of the VWP is considered more than ample for someone of working age). Then, if she's declined for a B-2, she will experience difficulty using the VWP and may not be able to visit the US at all. If she makes repeated or lengthly visits on the VWP, she may also get flagged for concern.
The fact is, if you're not married or co-habiting, the options for accompanying a partner with a US visa are limited, not to mention financially draining, and it's worth knowing the reality of the situation before you make an expensive mistake.