My friend's husband is cheating and I know this

she probably knows as well... might as well spill the beans...

She us a very emotional person, she might hate me or do something stupid. I am scared.

Don't tell her; she'll only shoot the messenger.

Rather, tell him. And say you know and, if he doesn't tell her, then you will.

Why am I suggesting this course of action?

She may very well already know, so by telling her, you're only pouring salt in the wound. By the two of them communicating about it, they can make a decision-- to divorce, to stay together, to continue with the status quo, or whatever.

But it's their decision to make.

By telling him, you push them into a decision (if they haven't made one already). By telling her, you are improperly injecting yourself into that decision.

Thank you.