Firstly, this is clearly a personal matter and not related to any law. No legal action is caleld for as there is no cause for action!
Now, what I don't understand from your post is, what do you mean by Show Cause? What does that Ex-Boyfriend wants? What is your present Girlfriend's stand on this?
As stated earlier, you need to get clarity from within yourself first about what you want. If you and your GF are united and think same way, then the Ex has no business to speak to you or her. If he harrasses you and your GF then you are at liberty to file a police complaint of harrassment on behalf of your GF.
Hope it helps!