Was this in CA? Because CA is a “right to work” state, which means fire at will, at any time. A signed offer that doesn’t include explicit contractual guarantees in the case of termination is worthless.
A “right to work state” is one in which the state has the authority to decide whether employees can be required to join a labor union as a condition of their employment.
California is an at-will employment state. At-will employment means that an employer can fire an employee for any reason or at any time.
If you resign from your job, you can file for some damages. However, she points out that she was between jobs anyway, so that's not on the table.
I'm really surprised that Stripe didn't actually offer her some money along with an NDA. This is kind of a big misstep.