If she doesn't want him talking about how good she is or is not in bed, that's absurd. She shouldn't have slept with him then. Tough. But there are certain things that need to be held close to the vest. If he were to share personal information about her that could lead to her losing her job, or home, or things like that, that's libel. If he goes all over the web and posts photos of her in vulnerable situations, then makes horrid comments about them, that's slander.
The first amendment covers ALMOST everything, so she is going way too far in her claims, but that doesn't mean everyone is legally allowed to say whatever they want whenever. Words do effect your life, your job, your reputation, and to use intimate knowledge of someone with the intent of damaging their reputation to the tune of a job or monetary loss is illegal.
That being said, he can say whatever he damn well pleases as long as the end result isn't a job or monetary loss on her part. So she's SOL on a lot of other things. She should have chosen better for a partner if this is her concern now, and she also could have chosen not to share certain pieces of information.
This goes to the first amendment, Free Speech. Nick Cannon can say whatever happened in that relationship if he wants to, Mariah has no grounds from stopping him from doing so unless it turns slanderous. The beauty of free speech is that it gets a point across weather people like it or not.
You can't muffle someone from talking about anything! Freedom of speech is a constitutional right. Some of these diva's think they can just do whatever while they are in the spotlight and have no one hear about it. Privacy is something we all want, but she can't force someone who was a big part of her life to be quiet. NO.
There's no way she should be able to keep him from talking about their relationship. He was part of it too. She isn't the only one who has rights in the relationship, he was effect by it all too. He should definitely be allowed to talk about the relationship if he wants to.
There is a thing called freedom of speech. Nick Cannon should be able to talk about the relationship. There is no reason he should not be allowed to talk about the relationship. There is no real law that can stop him from saying almost anything that he wants. If he agrees to not talk fine, but otherwise he can.