How to be a good/better friend ?

Hello guys, I need help about a serious matter: love and relationship.

For a better understanding of my behavior, here are some points about myself:
- I don’t have any experience in love,
- I don’t have many friends,
- I’m more like a side character to whom friends tell their love stories,
- I’m really realistic and logical concerning this matter,
- I would never date a married man (a big no no for me), I think it’d be disrespectful regarding myself, the wife of said married man and their children.

So, here’s the story:
I have a childhood friend who’s dating her previous college teacher. He is married with two children. He likes her, she likes him, I hate him (whereas I never met him).
In the beginning, when we were in our second to last year in college (not in the same major and university/college), she would tell me about this teacher who became her university tutor for her internship and master’s thesis. Everything was fine, until she told me about how close they had become in a short amount of time. They had exchanged numbers and were texting without respecting the teacher-student status (he began first she told me). I told her that it was weird and that I hoped she would be careful and restored some distance in their relationship. She didn’t tell me whether she was interested in him or not, I assumed she wasn’t until she told me a few weeks later that they had seen each other for a drink (and a little more), which confirmed her interest for him.

I was disappointed in her honestly, she knew he was married and was careless about the whole situation. I mean he was biased towards her, as he was her tutor and wasn’t as involved with his other students that he was with her. Besides, I was scared for her, what if her classmates and friends in her university knew about this or even the teachers ? She had to write a master’s thesis and he helped her more than he should have and was one of her teacher… In my opinion, the consequences could have been terrible.

Some facts about this teacher-married man: almost 40, has 2 children, said he never experienced anything like this before, he’s quite nonchalant about the whole thing (knows it’s his fault, he shouldn’t have but still carries on with my childhood friend…tf), will not divorce even though he said that his wife and him are no longer a thing...

I sincerely think that he has a midlife crisis and that my childhood friend is just a past-time.

Then, I asked my childhood friend to stop this relationship because I was worried of the consequences for her as a student and as a young woman.

She told me she would but lied during several months. While she was telling me that everything was back to normal, their relationship deepened and became more serious. He even introduced her to his children (not as a lover, as a student she said). Because I was disappointed and hurt, I was quite childish, I didn’t want to hear about him, so she and I didn’t speak for quite some time, until she contacted me and then I told her what my problem was (aka him). We did as if he didn't exist.

Now, she’s no longer a student, they’ve been together for almost 2 years. So she might not be a past time…but the thing is that he’s still married and will not divorce nor end anything. I think he’s a very selfish man and I want to punch him.

Furthermore, she’s not happy. As expected, she wants more of him now and became jealous of his wife/family life. Yet, she’s unable to leave him, she is too dependent.
I honestly wonder “why him” ? Is he the paternal figure and boyfriend she was looking for ? Did she feel lonely at some point to be this dependent ? (unlike me she’s quite surrounded, I know it doesn’t mean anything, but still…) Did she hold onto him because she thinks she’s unable to do anything without him as he helped her a lot previously ?

During her birthday, he left for a weekend to have a little holiday with his family. She’ll never be his number 1, except when he wants. That’s why I asked her to try to be less dependent, by texting him less.

I have this annoying role of rambling by telling her what she doesn’t want to hear/do, because I care and want her to be happy.

What would you do if you were me ? Would you let her be as it’s her life, knowing that she’s not happy ?
Can you relate ? She knows I want them to be apart, but am I right to behave like I did ?

I feel guilty to be this annoying.

Thanks !