Wendy, I feel for YOU as the friend. Our dear friends, a couple, went through the same thing a decade ago, with the husband walking in the door and announcing he "didn't want to be married anymore," with no earlier indications of unhappiness and a total unwillingness ever to work things out: "That won't change my mind so we're getting divorced." It's devastating to the couple's friends, too, especially if you think you're close to both partners in a couple and find that one of them is nothing like everyone had thought. So take some time to process this for yourself because it will hit you hard too.
Your friend needs to get a good divorce and custody attorney right now. That will be hard for her to consider because she is grieving and hurt and angry, and she may believe they can work it out, but even if they do eventually stay together, she should protect herself and her children right now and get good legal advice from someone experienced with divorce and custody. Her husband may say now that she can have the kids, but he could come back any time and make demands. Getting a lawyer may seem to her to be an admission that everything's over, but she must protect herself, her financial status, her kids and her future alimony or child support money. Going without child support isn't an option --I've seen that too, and it's cheating the children.
Besides helping her get legal help, which may be tougher because you're all overseas, just offer to talk but also provide more concrete help. Offer to take the kids for a few hours so she can get out of the house alone, or so she can see an attorney, or just so she can clear out the husband's stuff without the kids seeing her get upset. If you know what the kids like to eat, tell her you'll bring over food for all of them; maybe organize a rota of meals that you and the other friend bring. She surely doesn't feel like cooking or doing housework right now.
If you just say, "What can we do?" lots of people will say, "oh, that's so nice, but nothing." They hate to ask for help because they already feel helpless. Don't be vague, offer specific help and turn up and do it (unless she is a person who might get offended, as her friend you can figure that out).
I really feel for all of you in this situation I've seen before. I hope she gets good advice and doesn't cling to the idea of getting back together if he truly isn't going to try. Yes, it's good to work it out and she should push for that on the basis of their kids and their history together, but there does come a point when she may have to let it be over to protect her heart as well as her family and finances. Take care.