He should consult a lawyer and see how bad it will be for him. He should also sink some money into having a private eye follow her around for a month or so and make sure she’s not fucking around on him. If she is, just use that as blackmail that you’ll show the kid if she tries to fuck with you in the divorce.
If that’s not the case and they just grew apart, try counseling and get back on the right track. Shit thing is, women aren’t made to work as hard as men, they’re not set up to deal with the stress of it after they have children. If he can swing it, he should let her ease back and make less money. I essentially let my wife retire after our second child. She works for me/us now a few hours a day but it’s a gift for our family that she’s able to get them to and from school, play a little tennis, work a few hours and have all the household things done. Housework is a full time job if done right. Does he work out and keep himself in shape? It sounds like they’ve been in a rut for a long time and have just diverged to a point where they aren’t even “together”. He needs to date and continue to court his wife. She’s probably depressed from too much work at a late period in her life. Again, all of this sympathy goes out the window if she’s cheating. Then you go scorched earth.