The first thing he needs to do is protect his children. If she is being neglectful, then it is his responsibilty to ensure that his children are properly cared for.
The family law attorney is probably the best advice. Find out exactly what isproof is necessary for one parent to obtain custody and another parent to lose it. If he is not interested in contniuing his marriage (and quite frankly, I wouldn't blame him if he wasn't) then he needs to begin to make preparations now for that time, he needs to have proof of how he would care for his children should he obtain custody etc. It's hard for him right now, because of all the emotions involved...now is the time for him to do his homework and get his ducks in a row.