Don’t be a rat. How do you know that husband isn’t also cheating? Many polyamorous couples these days. Sometimes, women cheat to get back at their husbands for cheating first.
Also, how well do you know the husband? Does he carry a gun? The best thing for you to do is advise your good friend to stop. One, there’s disease if they are a polyamorous couple, and two, if they are not, the husband could lose his mind and do something more crazy, instead of just filing for divorce. Maybe your friend is tough and can defend himself. What if he defends himself so well the husband gets hurt? Jail time, anything could happen.
Best thing, keep your mouth shut and have your good friend understand the potential consequences of his actions. Plenty of single women out there.