The trouble is op that there are a great many of us on here who are the mum in this scenario.
Lots of us have had exes and their new wives blaming us for things that were not our fault, misinterpreting events, thinking the worst of us, believing half truths from teens trying to talk themselves out of trouble.
So I read it and thought - she probably told him to set his alarm, or tried to wake him several times but he kept rolling over and going back to sleep, or she forgot what time he was leaving.
Whilst I can objectively see that she may indeed be a terrible mother, who cheated on your dh, who intentionally allowed her child to miss a day out, who sells his phones to pay to get her hair done or whatever, I know all of that could be true, I can't help wondering what her version would be.
My advice would be to draw a line under it, collect him, have a good weekend, maybe resist moaning about his mum and ask him why he didn't set an alarm.
And btw - by 14 we are not organising half term activities for them. They are making their own arrangements.