My son in law has walked out

Nannyto3, how terribly sad for your daughter and how worried for her you must be. I agree that the best way forward is to say little about what a sh*t he's been and concentrate on encouraging your daughter to put her personal feelings to one side when it comes to the relationship between father and children. It will be really difficult for her to be magnanimous but their needs are so important and, if he turns out to be a cr*p father as well as a husband, it will make the children far more likely to turn to her for support if she bites her tongue at this stage.
One of my friends went on a 6 week course about divorce when her marriage broke up. She found it helpful to find out about the legal side of things, to talk to others in the same position in a safe environment and to just have general support from people without a loyalty to one side or the other.

It sounds like your daughter is actually the lucky one as she is rid of a bloke who turned out to be not very nice. His new partner has also shown her true colours too by getting involved with a married family man so it sounds like they deserve each. It probably won't last long either once she gets pregnant as he may become a serial bolter..

I hope she will be able to see this in time. How he behaves towards his children and their joint finances will set up their relationship with from now on. No one has to look at facebook so why use something which you all find upsetting.

Cliches are cliches because they resonate with so many situations we have all experienced...
nothing wrong with using a cliche... or just a hug... or shared anger / tears...

How you protect grandchildren at such a vulnerable age (another cliche... sorry) is perhaps best with hugs again... and consciously not slagging off their dad... they will do that enough themselves...
Perhaps you could lighten such emotional moments by looking at them and saying... well you would say that wouldn't you.. with a smile...

so sad for you all ...

we got through and eventually laughed about a charming walter mitty character who married our child whilst having another marriage in the USA... that was tricky too..

The same think happened to one of my daughters, they had no children at the time. I would also advise that you don't criticise her husband either to her. If he has left quite recently they may well get back together, as did my daughter and her husband. If you have bad mouthed him to her she may be reluctant to confide in you if their relationship looks like getting back on track. (just despise him in secret).

Bad enough to leave them, let alone boast about it on FB! What a sh.. But don't say any of that! This happened to a friend of mine. They were all decimated, but a few years on and things have sorted themselves out. The GC have come to accept it and still see their dad on a regular basis - I think that is important if you can all get it to work. Stay strong.

Why did the daughter look on his social media pages? Surely that's the last thing you'd want to do if your husband walked out.
If you want to twist the knife that he stuck in your back...then looking at what he's up to on social media is the way to go.
It's a rotten thing to have happened but I would have feigned disinterest no matter how I really felt.
Anyway, the OP needs to do now what every mother would do and that is to be as helpful as she can without taking sides.
I think that's the general consensus among GNers.

CAB can help guide her with the minefield of splitting assets and his future contributions.
A divorce need not be costly and, as he was the one to leave, she has the grounds to start proceedings.
It's only costly when there are massive assets and trusts involved, which won't be the case in this instance.

We know what you are going through. Our DIL walked out with children. That was 2 years ago, we think she has a mental problem. She yells, at stupid things and her Parents are no help, when she needs the help.

She didn’t look on his social media page deliberately- a colleague found it and showed her. How humiliating is that?!

Its more of a humiliation to him and his new girlfriend, although that's little consolation to your daughter.
Incredibly insensitive of him.

You sound like a really nice caring mother and I am so glad that your daughter has turned to you for support. Please don't criticise her husband especially in front of the children as she will feel forced to defend him. Don't take over the children unless she asks as otherwise she will think she is failing as a mother as well as a wife. I speak from experience as my mother was awful when my husband left me.

Keep your daughter close and let her get over this in her own time I feel so sorry for her but she will come out the other side in time,this happened to my DD six yrs ago and she nearly lost her home as he had stopped paying the mortgage without her knowledge but she got over it and has come out the stronger person.
Posting his love life on Facebook is something I would expect from my 14/15yr old grandchildren not a grown man the world has gone Facebook mad.

Hopefully his new 'girl' is as childish as he is: it sounds it, posting it on Facebook. I feel furious on behalf of your daughter and children.