Congratulations on the visa, but then, with hindsight, of course there was not going to be a problem.
Arrive before visa start? If she is US, and therefore not a Visa National, she will encounter no problem boarding the plane. The plane could of course be delayed a bit, and it probably would not be a good idea to get to the head of the queue to speak to the Immigration Officer before the start of her visa ..... but can she help it if she needed to use the toilet as soon as she got off the plane?

She is landing at Heathrow? The queues for non-EEA/Swiss can be quite long so no way she would need to loiter in a corridor until Midnight.