Oh, wow. Poor cousin.
Honestly, I feel that she needed to be told already, other issues aside. And another poster beat me to it-this is an in-person conversation. Chances are, her jerk of a husband already has told her some lie. His demeanor kinda freaked me out even from here. To address the etiquette and moral aspects, here are my points:
1. Your friend should do this in person. Over the phone means well, but from an etiquette standpoint, this just seems more polite.
2. Have her invite cousin to *her* house, or to a middle area, so it's neutral ground. Being told in her own home, where CH is "present" even if he is not there might lead to her defending him. Either way, the messenger should be prepared to be shot., at least initially.
3. She should stay calm and reassuring and be prepared with a back-up plan. What will cousin do, or go, are questions that might come up.
4. Chances are, this creep will wait until she is most vulnerable and then leave, or spill this. At least NOW, she has options. As another PP said, she can take a job out of state, sell the house, what have you--in short, she can think of *her* needs, not the couple's needs.
5. Finally, she might need back-up if she decides to leave. This guy sounds like a potential abusive jerk, with his menacing attitude towards your friend. Or he might try to manipulate her into staying. Either way, she will need help--even the strongest of women need a little additional steel for their spines sometimes.
Best of luck to all of you and please update!