If you travel in the same social circles, you might want to avoid being in the same place as your ex and the friend you want to date.Really, though, you probably shouldn't be hanging out with your ex in the first place.
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The guy might want to wait a bit before starting anything with you out of courtesy to his friend. Also, be prepared for him to not want to get involved with his buddy's ex at all.) But you don't owe your ex anything. Even if he's jealous, he'll have to get over it eventually.
If your break-up is civil and you're still friendly, maybe give it a couple of months.
Be honest, and as cool as possible about it, and hopefully he won't be a big jealous baby for too long.
Ideally, the guy isn't your ex's best friend. The problem could occur once you start hanging out your ex and the new guy.
You'll have a much better shot at being truly heard if your reasons come from the heart.