My friend is in trouble

Hi people!
Well, this is my very first entry so i don't really know howshall I start off.
I guess I will start off my sharing with you about the situation my friend is in.
My friend is 18 turning 19 this year has started working at this dental clinic after her A level. She met this really cute dentist who has hit 30 years old already and married. Initially, when she mentioned about him we just thought that there was no big deal about it. After all it never went into our head that our friend would have something more to do with this married dentist. 9 months has passed since she first started working at that dental clinic. Recently, I was talking to her regarding this matter about quiting her job since she started schooling already. The more we shared, the more I learn about things between this dentist and her. Well, in fact I know about these things from my other friends and got worried. So i sort of lead her into telling me. I started telling her to be becareful, all in all he is a married man. I got even more worried when I found out that they are already holding hands and going out once a week. Being her good friend, I thought I have the responsibility to tell her that if something were to happen, she will be the one getting hurt as well as the dentist's wife. She said she wont be and that she doesnt care about his wife because all she knows is that he is really important and she will not leave him. The more I say, the more defensive and agitated she became. So i ended off telling her to be happy since she has already decided so firmly on it.
And because of this, she is rather pissed off with me. So it will really be helpful, if you people out there can help comment on it.

thanks!