I'm going to date my ex's friend.
Why would you want to date your ex's friend?
My ex was always cold to me, but his friend always treated me so nicely.
There's no real reason to date a friend of your ex. It will only make the relationship a lot more complicated and make it hard to have a healthy and meaningful love life.
I'm just going to go ahead and do it, I'm going to talk to him this week.
If you're going to do it I suggest you talk to your ex about it. You're being inconsiderate to both your ex and their friend because you will put them in an awkward situation with each other. At the very least think about how that will affect your ex's friend if not how it affects you and your ex.