Help a girl out please :)?

You really should first check if your best friend has TRULY gotten over him, not just saying she has. If she really has, then casually say something like, "Wanna grab a slice of pizza later?" or something similar. If you can't do it in person, call him, chat on the phone for a while, then ask. It's easier to not get rejected in person. If he says no, try to make a joke out of it so it doesn't seem awkward.