As you've been told, it will be VERY risky for her to travel during the curtailment period. I personally wouldn't do it.
She can't re-enter on her Tier 2 as it'll no longer be valid. And if she lies and pretends it is, well, that could set her up for a deception charge with up to a 10 year ban from the UK.
She could attempt to re-enter as a visitor. To do so, she would need to prove that she is in fact a tourist and not trying to live in the UK to bridge the gap between visas. The only way to ensure she would be successful with this route would be to apply for a visa in advance - which she absolutely CAN do. However, she needs to apply from the USA.
What she will need to be successful in a visitor visa application:
- return flight booked to the US
- proof of a job to return to in the US (letter from employer granting leave and date required back at work)
- proof of a home that she will be maintaining in the US (lease or mortgage)
- sufficient funds in her bank account to support her trip
Those are the main bits. She may be fine and have no issue at the border, but being that her Tier 2 is no longer valid, she will be open to more scrutiny.
Have you two lived together for a minimum of two years? Or have you considered marriage? I know that no one wants to go down that route because of immigration, but sometimes...

x-posted with Sirius