First of all let him know he is not alone. This has to be devastating. Second tell him to hold those kids close as they share the same pain. The lawyer is a good idea. Without way more details that you would want to give over the internet I can’t do much for him or you other than some common sense basics.
1.Get lawyer /Get custody
2.Change locks, home, car ect
3.Try and keep routines as normal for children as possible
4. Inform school if kids are school age.
5. Find family, or friends who can help while he figures everything out. (grandma, cousin, aunt, any warm responsible body)
6. Close all accounts she has access too, bank, credit ect
7. Take all the kids in for a check up by pediatrician today.
His first job is to protect his family. His kids are now the focus of that family. This way he protects the home they live in the money that pays for it to provide for them as best as he can.