It feels like her role is an auxiliary function, like Office Manager or QA, so I think her generic analysis works. If her role was central to the business, then I don't think she would be valuing her contribution with a one-liner like 'Employee’s value-add to the company (I used 15%, which I think is pretty low!)'. Instead the question of her value-add would be the starting/central point of the negotiation.
If she thinks that she's increasing the odds of a successful exit, then below market compensation could be financially worthwhile for that reason alone.
