Should we go with her since she's paying?

tell her you would love to go, but the timing is just bad for you. He has a job that he has to finish today before starting another one tomorrow. These people are depending on him to do the job he said he would do. If he disappoints them in order to go to an amusement park, this will reflect badly on him and they may give him bad feedback to others who might employ him in the future. The weather is possibly not going to cooperate. Maybe another time when the weather is better and there isn't so much going on would be the wiser decision.