No OT but end of project bonus?

My friend is distraught that her company is switching her overtime work from hourly to a lump sum at the end of the project that may or may not equal what she would have gotten (they said it would, but only verbally). She contracts for a federal client and her project is short-staffed, with her being the leading contributing employee.

Is this even legal? I've tried to convince her to quit multiple times, but she's one of those "feels bad for her co-workers" people and can't get over that, so as of now, threatening to leave isn't an option. Any advice appreciated since she's at her wit's end. Also posted this in r/consulting

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