The Northeast has a large number of airports and a lot of them with very high E-W and W-E traffic. Picture the normal sky as fenceless wilderness... add a storm like this and it becomes one giant fence with few openings. Effectively choking the system. FAA is pretty good at predicting problems reducing the number of diversions by keeping aircraft on the ground at the departure station.

After thinking about it longer I bet the gate/tarmac delay issue was more likely. F9 decided to route around the problem (very expensive with added fuel) and then possibly realized mid flight a tarmac delay was inevitable. You do have to give the experts that do this some credit... dispatching something like this could have just been solved with a cancellation and your friend would not have arrived albeit late the same day.