Gracesmum talks a lot of sense- and I have witnessed many colleagues who worked very long hours but not particularly effectively, and often to hide a sense of insecurity. This may well have absolutely nothing to do with this particular case, but it is a fact- and not just in teaching.
I was lucky in a way to go into teaching late- I went to Uni when our youngest started school. So I had to be absolutely organized about how I used my time, and learnt to teach very well and effectively, but without re-inventing the wheel every day. It really did concentrate the mind. One of our young teachers had a nervous breakdown after having her first baby. She was so used to have tons of time to prepare every lesson, make new worksheets, acetates, powerpoint lessons, games, individual activities for separate groups, for each lesson (secondary)... and when she suddenly didn't have the time to do that, despite increased experience, she crumpled.
How to organise one's time and select priorities is a huge factor in being successful, in any job with responsibility. As said, this is a general comment, and not about this particular case. Although it may well be a factor too.
What did your son-in-law do before?
Much too late now for the school to recruit someone- so what are the kids supposed to do in September without a teacher? Those rules are there for a very good reason. I am sure none of us would like to ever face the situation where our children and grand-children have no teacher at the beginning of the year.