Not an actual screenshot, but mocked up from what's in the book. Harry says it was done by texts and gives what was said in them. My understanding is that
Kate sent a message to Meghan saying none of the dresses fitted properly and what was going to be done about it.
Meghan got a lot of messages that day but her head was taken up by the will he/won't he about her father so didn't reply to Kate for 24 hours
Kate got stressed about the lack of a reply because all the time she heard nothing was time that could have been used to alter the dresses, and she knew just how badly they needed it
I can understand that Meghan was stressed. Weddings can be difficult to organise and the biggest wedding of the year even more so, without the added (enormous) complication of Thomas Markle, so I'm not surprised Meghan collapsed in a heap of tears at some point. What I think is wrong is to make out that it was all solely Kate's fault. It was the pressure of the whole thing, not that one incident. The crying could easily have been the fact that the dresses didn't fit and it was getting late in the day to do anything about it. But H&M want Kate to be the fall guy.