Is your friend reasonably good with forms etc?
If so, he could complete a Form C100 (available on Gov.uk). He will also need to complete a C1a as there is considered to be risk of harm to the children. These will need to be sent to the Family Court local to the children.
I have just started this process myself – forms submitted, it’s early days but I’m going to try and do as much as I can myself.
If he feels the children are at risk of harm, he should also contact children’s services immediately (unless they are already involved?)