Yet, when an employer changes the terms of an employee's contract their only option is to quit?
Different types of contracts. An “at will” employment contract allows the employer to change working conditions (to the point of simply letting you go) but also allows the employee to walk at any time.
If both sides agree to some form of penalty for changing/breaking the contract, then it’s no longer truly “at will” and both sides are bound to contract or face the prescribed penalty.
IF she has a contract - if she is in a union, for example - then the answer is no. However, most employment is "at will". There is no employment contract to speak of.
Unless an employment contract has a specified term (the military is the primary such example), employment is at will on both sides. An employee can quit without notice, demand a change in pay or working conditions, or otherwise insist on renegotiating or terminating the relationship without notice, although two weeks notice is polite and traditional. Would you eliminate that option?