Praise be to Allaah.
The fact is that you have delayed too long in asking about your problem and taking a firm stance concerning it.
It is true that the matter - praise be to Allah - has not reached an alarming stage as happens with many such relationships, but it has taken the first step on which what comes after is built. What we mean by that is the step of establishing the relationship between the two parties. The fact that they are at this age and are still young could lead to this relationship developing in the wrong direction.
We are not concerned about looking at the content of this chat which cannot possibly, based on the nature of the situation, remain innocent and be limited to discussion of religion, politics, and so on. We do not need to look at the content of this chat and whether this relationship has reached anything that may be regarded as haraam. The relationship itself from the outset is haraam in and of itself. No relationship between two young people at this age can remain within the boundaries of innocence; rather it will inevitably develop into an emotional (romantic) relationship the extent of which and what it may lead to cannot be known except by Allah. The Prophet (blessings and peace of Allah be upon him) said: No one of you is alone with a woman but the Shaytaan is the third one present. Narrated by Ahmad (114) and others; classed as saheeh by al-Albaani.
According to the nature of the relationship and the extent to which their conversation is private, the Shaytaan will base his hopes of tempting them to commit immoral actions.
We have previously spoken of the ruling on correspondence between the sexes in several answers. See for example the answer the questions no. 26890 and 93450.
What you have to do now is to stop your daughter continuing with this haraam relationship before it reaches the stage that it will be difficult to stop it.
Start by explaining the shari ruling concerning that to your daughter and tell her that it is not permissible for her to continue with this relationship.
If he is serious about his relationship with her, then it is possible to make it a legitimate relationship by means of marriage, if they both want that and he is a suitable match for her; otherwise what is the point of continuing with it?!
And Allah knows best.