Its difficult because they probably feel they can change the person and that if they let go, then they have failed and its their fault it ended, probably also blaming themselves that their partner cheated in the first place. The self esteem goes out the window and then they can't see them self as a person another man would want, so they choose to stay where they are.
Another thing is, people who are cheated on or told them should leave their partner, get very defensive about their partner, making excuses for them. Its a touchy subject to get on with these woman because it could easily get out of control and end up with you losing a friend.
The person has 2 choices, leave or stay, leave and accept the cheater for who they are, and accept cheating in the relationship (dont complain constantly they cheat bla bla, you are the reason you are still in the relationship that has a cheater). Leave and accept that you only have one life and the more years you spend with a cheater is wasted years, because you will regret not leaving when you are old and you cannot get your youth or the years lost back.
Its really simple, once a cheater see's you are staying with them, they take it that you accept their cheating ways, they wont stop, and anybody that stays in that relationship is a fool, you deserve and can find so much better, the trick is to get these people to see that for themselves, then they will make the right choice.