Can I tell my friend not to date a married man?
What do you think will happen if your friend decides to go out with him?
She will quickly break up with him and I think it's better for her to stick with the person she knows.
While it's generally frowned upon to interfere with a friend's love life, you are right to do so when your friend is trying to pursue a married person. Cheating is wrong and people should respect the marriages of other people. Trying to persuade your friend not to go after a married man is a good thing since it can help prevent infidelity and the possible breakup of a marriage.
What is wrong with those people and why they can't commit?
I don't know, and you are completely correct in saying that people shouldn't date if one of them is married. You would be helping your friend by attempting to prevent this.