Son, grandson, DinL

If the SiL has a highly paid job then he'll have to be prepared to " pay out ". Some lawyers make a bee-line in forcing money from high-earners until their pips squeak and if your son isn't careful he'll lose out, property-wise and also maintenance too.
Equality still doesn't always exist where the law is concerned.

DiL could be bi-polar due to perhaps PND which may have been left untreated ? I suggest she visits her GP as her unwillingness to work could be more of a depressive state considering that she has a degree. A job would do her good ! Someone who uses facebook ( another curse ) to berate others definitely needs treatment, as it's not normal.

We've been through a similar thing. Details of that aren't important here, just advice.
Yes, try to keep matters amicable, take legal advice but do as much as possible between themselves, make sure that the residence/visiting arrangements for your GS (our GC are 50/50 with each parent) are acceptable and that all the financial and other arrangements are tied up firmly. Some flexibility is good if it can be achieved.
Haven't forgiven our ex-DiL, but we remain neutral about her with the GC.
Given what you've said, I don't recommend reconciliation. Our DS took time to recover but is much happier now.

However much you dislike your DIL, it is important not to voice this too vociferously as sometimes couples do get back together and these words cannot be unsaid. Also you do not want her to stand in the way of you having contact with your GC. I do hope this can be resolved with the least rancour possible.

Funny thing about the label.....there are an awful lot of "narcissistic" MILs and mothers on Mumsnet!

It's possible your son and daughter in law may reconcile.
It's also possible they may be able to reach agreement about contact, residence and finances. I hope so because involving solicitors is expensive, though often very necessary.
Try not to let your dislike of your daughter in law be at the forefront here. She will always be your grandson's mother. Separation and divorce are always hard for children and the most important thing is that their parents do the best they can to be amicable, friendly and pleasant when they're together and when talking about each other.

Whatever the cause it's clear your son is unhappy in that marriage. They have been married for 13 years and any time cohabiting before the marriage will be added to that. That being the case, it's considered a long marriage and the normal starting point is a 50/50 share of all assets.

However, a priority for the judge will be a roof over the head of your GS, so that means a roof over her head too as I presume she's the main carer at present. However, the judge would also want to see that she is doing her utmost to find employment to maximise her contribution.

If your son is set on divorce he should seek legal advice, but also try to keep things as amicable as possible. The most important reason being so that his DS doesn't experience excessive conflict between his parents but also to save money. It's cheaper in the long run if they can sort things out between themselves then have it ratified by the court.

I can see how this is causing you a great deal of distress, but as Stella says, tread warily as to antagonise your DiL may have consequences with regard to access to your GS.

I agree BluBelle so many labels attached to behaviour now. As you say there do seem to be a lot of daughters in law with mental health problems out there.

Narcissistic is the new flavour of the month There seem to suddenly be many daughter in laws in this category
There’s always two sides to the story and you will only want to see the one, it’s natural but it may not be as one side-saddle as you see it

As a mother and grandmother, of course we always take our own children's side in these matters. It does sound as if your DIL has been a toxic presence in your son's life, and he has now moved on.

He needs to see a lawyer and to make sure everything is sorted with the upcoming divorce. Your DIL will certainly have to get a job now, since she can't expect to live off him any more ,and no divorce settlement would award her any maintenance money since she just has the one child who is at school.

Your son will also have to come to an agreement about custody of his son and maintenance for the boy. his lawyer can advise him about those matters.

In the meantime, the best thing you can do is to support your son but to keep out of any other matters involving your DIL. You don't want to jeopardize any contact that you want to have with your grandson - divorce can change your access to him so step carefully.

Best wishes to you.