@DjLlamacorn1 my first advice is to not retaliate. If you go after the bully then that gets both of you into trouble. Second advice is to just offer your comfort.
Third advice is to screenshot all of the posts the bully has made about or to your friend
Advice four: suggest to your friend to either block the bully, report them or to just log off for a while.
Advice five: report this to an adult and bring solid evidence. In some states or countries, cyberbullying is technically considered a cybercrime depending on the situation. If your friend is receiving death threats then that is technically considered first degree murder