With the start-up she would get to stay until the project is finished? Is there a timeline for the project, and after that project is wrapped up would she leave or would she be kept on to grow the Colorado office? Or would they let her go after the project is done?
Is the work at the consulting firm substantially different from what she does now, and is this a career change she’s excited about? If so, she should go with the new job.
She shouldn’t stay at a job just because she feels guilty for leaving if it’s not what she wants. She should make this choice based on where she wants her career to progress and the best choices to facilitate that. Think about the growth potential with either option.
On the other hand, if she loves the start up and is excited about the opportunity to work remotely and build the new office, then it’s possible (but still bad) to renege on an accepted job offer… Especially if she hasn’t put into motion the downstream HR stuff like relocation and payroll.