My new girlfriend's high school boyfriend often sends messages to harass her.

Maybe your girlfriend doesn't want to hurt that man if she doesn't work hard

It's okay to change the phone, or you can call that person and tell him.

Say that you are his date now, and ask him not to harass your girlfriend.

And tell your girlfriend the information and phone number of her ex-partner, and don't answer it or return it.

Ask your girlfriend to put herself in your shoes.

Your girlfriend's attitude is also very important to this matter.