Honestly, as soon as I saw the ages they were when they were married at the top of your post I was bracing for him to be controlling.
This is coercive control, and if she's been subjected to it since she was a teenager there will be a huge amount of brainwashing/conditioning in play here.
Do not try to tell her what to do or think or feel about it. Ask questions to get her thinking herself, and maybe reflect non-judgementally to give her space to open up (e.g. If she describes an incident "oh, I'd be really upset if that happened to me... How are you feeling about it?")
Don't criticise him, because she'll most likely pour her energy into defending him rather than having space to reflect on what you've said. It will probably also make her feel judged ("well, if DIL thinks he's so awful she must not think much of me for marrying him...").
There is a useful page on the Women's Aid website about how to support someone who is in an abusive relationship (which is what this is - it's coercive control).
It is very, very common for domestic abuse to escalate like this when the abuser retires.
You can't fix this for her, or fly in and save her, which I know is hard. Aim to be someone she can trust and rely upon, and to keep communication open - because abusers work hard to isolate.
If you increase your own knowledge of how coercive control works - the dynamics driving it, rather than a list of symptomatic behaviours to spot - it will help you to support her.
Women's Aid can help with that, but so can the Freedom Programme. You don't have to be in an abusive relationship to go on their courses. They are information not therapy, and open to all women.
If you ever reached a point where she might be receptive to attending the Freedom Programme herself, it could be what helps her find a way to cope or free herself. They will never tell her to leave him (and nor should you) and they won't judge her.
If you had attended the course yourself, it would possibly be an easier way to raise it with her in the future - "I went on this interesting course, I wondered if you might find it useful..." Kind of thing.
When I started the course the first time (and I was in an abusive relationship) I didn't understand why I had been referred to them or why I was there. I was convinced it wasn't relevant to me and was all a big misunderstanding. And then I listened and realised they were describing my life. Which was a huge shock - and some women aren't ready to hear it yet and don't come back to finish the course until much later, if at all. So be prepared for that.
But they said at the beginning they weren't telling us what to do or to think, and they weren't judging us or our relationships - they just wanted to share information with us so we could see if any of it was useful and then decide for ourselves what to do with it.
That's how it needs to be framed for your MIL too. She's lived her whole adult life with someone controlling her, so she will be very tuned in to any experience of other people seeming to do the same. You need to be different. You need to be someone who is not telling her what to do - either explicitly or manipulatively.
Show her that away from him, things would be different. But be aware, if you've lived that much of your life being controlled and prevented from learning how to be independent, the idea of existing in the world alone and without control to contain you can be absolutely terrifying and insurmountable. She may never feel able to leave, and may miss him and return if she does.
All of these things are normal reactions to abuse.
Www.freedomprogramme.co.uk
You can speak to Women's Aid in 0808 2000 247
Do you have support yourself for coping with this?