No, you shouldn't get jealous. The key here is that you used to date the guy. His dating habits are none of your business. (And your dating habits are none of his business, either.) Being jealous is just going to make you miserable.
But it might be awkward watching your best friend pursue the guy you used to date. If it's too painful for you, you may want to ask your friend to do it when you're not around. Or, you may want to hang around with other people for a while.
You've got new adventures ahead. New guys to date. Don't put your focus on this guy -- he's ancient history.