Wow! Yep, that young woman dodged a bullet! Not necessarily so much from the mother-in-law but her son who comes across as passive aggressive and self-pitying.
So the MIL has some sad feelings about it and that's completely okay, we get to feel what we feel whether it's acceptable or not and she wisely kept her mouth shut, so well done her for that.
What actually irked me the most about her letter was the reference to the woman becoming pregnant when her son had "been through a lot over the last few years and he didn’t need this" but was prepared to take "responsibility" for the child! As though he is doing something noble by choice!! FFS, he was just as responsible for her becoming pregnant! I've been through hard times when a pregnancy would have been disastrous timing and you know what, I made damn sure I used contraception!
There is a clear sense that this MIL simply doesn't view her son as an adult who made his own choices.