The questions post in a queue and free lance experts answer as they are either available or willing to do so. It is not like a chat site.
Now back to your question-
It may not be appropriate to approach the boys parents. That would cause resentment from your daughter and lead to trust issues. It can also cause the two of them to feel a sense of shame (although the act itself is not something to shame for, it is their age that you are concerned about)
You would want to first address this with your daughter somehow (if you had read her messages without her knowledge, she may become angry and not listen to what you have to say)
The approach you use with her would be of importance. You want her to trust you and to respect your feedback. Perhaps try to solicit information from her as far as how things are going. She's dated the same boy for quite some time now (which is somewhat unusual for a young teenager) Provide some education on sexual behavior (she may have gotten some knowledge from the internet and her friends already)
Even if they halt their relationship, unless you are able to watch her 24/7, she would still do what she wants to do. Your goal would be to let her see that her behavior may have negative consequences. Otherwise, she will do what she wants just to prove that she can (teens are very egocentric) When you talk to her, remain calm, non accusatory and ask open ended questions so she does not feel blamed or put on the spot.
Yes. Setting boundaries and giving out consequences is the right thing to do. She could still communicate via the internet so taking the cell phone may seem as a temporary solution. As far as dating, that would be at your discretion (she could still have guy friends, the issue you're having is not to engage in sexual activities which is reasonable)
You could offer some flexibility to her if she is able to prove to you that she is able to follow directions and to respect your suggestions. If she is not, then you point out to her that is a sign of her immaturity level and reinitiate to her that is exactly why you have those rules.
Speaking to the boys mother is all right. She may have some idea of how to approach this as well. Two heads working over it is always better than one.