At first she’s so supportive and nice and says all the right encouraging things. She swears once again that nothing is happening between them, but you don’t believe her. A few days later, you see her write on his Facebook wall and you’re so mad you don’t know what to do.
Then, out of nowhere, she starts to get weird about it whenever you bring up his name. A few weeks later, you find out (somehow) that she’s been really friendly with your ex.
This will destroy trust, and with it any chance of maintaining the friendship. There is probably a reason that your friend and the ex broke up in the first place, and over time your friend will most likely realize that.
Maintain your distance: Same advice, different meaning. Don't force them to come to that conclusion any sooner than they are comfortable with.
The world is divided into two groups of people: those who think it's OK to date a friend's ex and those who believe it's completely off-limits.
When you and your friend are both in the "OK" camp, it can work if they date your ex, or you theirs.
Your new partner (your friend's ex) should as well, but remember that they have broken up, while you and your friend are still "together." An explanation of how important your friendship is to you and a sincere acknowledgment of the awkwardness of the situation might help.